Simplified ExpressiveFor

docs/guide/migration-from-projectables.md

@@ -161,10 +161,12 @@ public string FullName
 
 See the [Projectable Properties reference](../reference/projectable-properties) and the [Projection Middleware recipe](../recipes/projection-middleware) for the complete feature.
 
-**Option B -- `[ExpressiveFor]`** (separate stub, supports cross-type mapping):
+**Option B -- `[ExpressiveFor]`** (separate stub, also supports cross-type mapping):
 
 **Scenario 1: Same-type member with an alternative body**
 
+Use the co-located form: a property stub on the same class combined with the single-argument attribute. `this` is the receiver naturally -- the migration reads almost identically to `UseMemberBody`.
+
 ```csharp
 // Before (Projectables)
 public string FullName => $"{FirstName} {LastName}".Trim().ToUpper();
@@ -178,8 +180,8 @@ using ExpressiveSharp.Mapping;
 
 public string FullName => $"{FirstName} {LastName}".Trim().ToUpper();
 
-[ExpressiveFor(typeof(MyEntity), nameof(MyEntity.FullName))]
-static string FullNameExpr(MyEntity e) => e.FirstName + " " + e.LastName;
+[ExpressiveFor(nameof(FullName))]
+private string FullNameExpression => FirstName + " " + LastName;
 ```
 
 **Scenario 2: External/third-party type methods**
@@ -212,9 +214,9 @@ static OrderDto CreateDto(int id, string name)
 
 | | `UseMemberBody` (Projectables) | `[ExpressiveFor]` (ExpressiveSharp) |
 |---|---|---|
-| Scope | Same type only | Any type (including external/third-party) |
+| Scope | Same type only | Same type **or** any accessible type (including external/third-party) |
 | Syntax | Property on `[Projectable]` | Separate attribute on a stub method |
-| Target member | Must be in the same class | Any accessible type |
+| Target member | Must be in the same class | Co-located (single-arg form, `this` is receiver) or cross-type (two-arg form) |
 | Namespace | `EntityFrameworkCore.Projectables` | `ExpressiveSharp.Mapping` |
 | Constructors | Not supported | `[ExpressiveForConstructor]` |
 

docs/recipes/external-member-mapping.md

@@ -191,23 +191,47 @@ static class DateTimeMappings
 |------|-------------|
 | EXP0014 | `[ExpressiveFor]` target type not found |
 | EXP0015 | `[ExpressiveFor]` target member not found on the specified type |
-| EXP0016 | `[ExpressiveFor]` stub method must be `static` |
 | EXP0017 | Return type of stub does not match target member's return type |
 | EXP0019 | Target member already has `[Expressive]` -- use `[Expressive]` directly instead |
 | EXP0020 | Duplicate `[ExpressiveFor]` mapping for the same target member |
 
 ## Tips
 
 ::: tip Match the signature exactly
-For static methods, the stub parameters must match the target method signature. For instance members, add the receiver as the first parameter (e.g., `static string FullName(Person p)`).
+For static methods, the stub parameters must match the target method signature. For instance members you have three options: write a `static` method stub whose first parameter is the receiver (e.g. `static string FullName(Person p)`), write an `instance` method stub on the target type itself where the stub's `this` is the receiver, or write a **property stub** on the target type (parameterless; `this` is the receiver) -- property stubs can only target other properties.
+:::
+
+::: tip Co-locate when possible
+When the target is on the same type as the stub, you can drop `typeof(...)` and use the single-argument form -- it targets a member on the stub's containing type. Combined with an **instance property stub**, this is the ergonomic shape for the case where a property has its own backing storage (a plain settable auto-property used for DTO shape, serialization, or in-memory assignment in tests) but you want queries to compute it from other columns:
+
+```csharp
+public class Person
+{
+    public string FirstName { get; set; } = "";
+    public string LastName { get; set; } = "";
+
+    // Regular auto-property — can be assigned directly (for DTOs, tests, deserialization).
+    public string FullName { get; set; } = "";
+
+    // When used in a LINQ expression tree, FullName is rewritten to this body,
+    // so EF Core projects it from the underlying columns instead of trying to
+    // map it to a column of its own. `this` is the receiver automatically.
+    [ExpressiveFor(nameof(FullName))]
+    private string FullNameExpression => FirstName + " " + LastName;
+}
+```
+
+A method stub (`string FullNameExpression() => ...`) works the same way and is appropriate when the target is a method or you need a block body.
+
+If the property has no backing storage and the same body works at both runtime and query time, put `[Expressive]` directly on the property and delete the stub.
 :::
 
 ::: tip Consider [Expressive] first
 Many `[ExpressiveFor]` use cases exist because of syntax limitations in other libraries. Since ExpressiveSharp supports switch expressions, pattern matching, string interpolation, and more, you may be able to put `[Expressive]` directly on the member and skip the external mapping entirely.
 :::
 
 ::: warning Placement
-`[ExpressiveFor]` stubs must be in a `static` class. The class can be in any namespace -- it is discovered at compile time by the source generator.
+`[ExpressiveFor]` stubs can live either in a `static` helper class (with a `static` method stub) or on the target type itself as an instance method or property. Either way, they are discovered at compile time by the source generator.
 :::
 
 ## See Also

docs/reference/diagnostics.md

@@ -6,6 +6,10 @@ The ExpressiveSharp source generator and companion analyzers emit diagnostics du
 See [Troubleshooting](./troubleshooting) for symptom-oriented guidance -- find the error message or behavior you see and get step-by-step resolution.
 :::
 
+::: info Retired diagnostics
+`EXP0016` ("`[ExpressiveFor]` stub must be static") has been retired. Instance stubs on the target type are now permitted; constructor-stub and unrelated-type mismatches surface as `EXP0015` (member not found) instead.
+:::
+
 ## Overview
 
 | ID | Severity | Title | Code Fix |
@@ -25,7 +29,6 @@ See [Troubleshooting](./troubleshooting) for symptom-oriented guidance -- find t
 | [EXP0013](#exp0013) | Warning | Member could benefit from `[Expressive]` | [Add `[Expressive]`](#exp0013-fix) |
 | [EXP0014](#exp0014) | Error | `[ExpressiveFor]` target type not found | -- |
 | [EXP0015](#exp0015) | Error | `[ExpressiveFor]` target member not found | -- |
-| [EXP0016](#exp0016) | Error | `[ExpressiveFor]` stub must be static | -- |
 | [EXP0017](#exp0017) | Error | `[ExpressiveFor]` return type mismatch | -- |
 | [EXP0019](#exp0019) | Error | `[ExpressiveFor]` conflicts with `[Expressive]` | -- |
 | [EXP0020](#exp0020) | Error | Duplicate `[ExpressiveFor]` mapping | -- |
@@ -417,32 +420,6 @@ static double Clamp(double value, double min, double max)
 
 ---
 
-### EXP0016 -- Stub must be static {#exp0016}
-
-**Severity:** Error
-**Category:** Design
-
-**Message:**
-```
-[ExpressiveFor] stub method '{0}' must be static
-```
-
-**Cause:** The stub method is not declared `static`.
-
-**Fix:** Add the `static` modifier:
-
-```csharp
-// Error: not static
-[ExpressiveFor(typeof(Math), nameof(Math.Clamp))]
-double Clamp(double value, double min, double max) => /* ... */;
-
-// Fixed
-[ExpressiveFor(typeof(Math), nameof(Math.Clamp))]
-static double Clamp(double value, double min, double max) => /* ... */;
-```
-
----
-
 ### EXP0017 -- Return type mismatch {#exp0017}
 
 **Severity:** Error

docs/reference/expressive-for.md

@@ -10,15 +10,21 @@ using ExpressiveSharp.Mapping;
 
 ## How It Works
 
-You write a static stub method whose body defines the expression-tree replacement. The `[ExpressiveFor]` attribute tells the generator which external member this stub maps to. At runtime, the replacer substitutes calls to the target member with the stub's expression tree -- call sites remain unchanged.
+You write a stub member -- a method **or** a property -- whose body defines the expression-tree replacement. The `[ExpressiveFor]` attribute tells the generator which external member this stub maps to. At runtime, the replacer substitutes calls to the target member with the stub's expression tree -- call sites remain unchanged.
 
 ## Mapping Rules
 
-- The stub method **must be `static`** (EXP0016 if not).
-- For **static methods**, the stub's parameters must match the target method's parameters exactly.
-- For **instance methods**, the first parameter of the stub is the receiver (`this`), followed by the target method's parameters.
-- For **instance properties**, the stub takes a single parameter: the receiver.
-- The return type must match (EXP0017 if not).
+- The stub can be a **method** (receiver supplied as the first parameter for instance targets, or `this` for instance stubs on the target type) **or** a **property** (parameterless; `this` is the receiver for instance stubs).
+- The single-argument form `[ExpressiveFor(nameof(X))]` is shorthand for `[ExpressiveFor(typeof(ContainingType), nameof(X))]` -- use it when the target member is on the same type as the stub.
+- For **static methods** (and static stubs over static members), the stub's parameters must match the target method's parameters exactly.
+- For **instance methods** with a `static` stub, the first parameter of the stub is the receiver (`this`), followed by the target method's parameters.
+- For **instance methods** with an `instance` stub on the target type, `this` is the receiver; remaining parameters match the target's exactly.
+- For **instance properties** with a `static` method stub, the stub takes a single parameter: the receiver.
+- For **instance properties** with an `instance` method or property stub on the target type, the stub is parameterless.
+- For **static properties**, the stub is parameterless.
+- Property stubs can only target other properties (no parameters to carry method arguments).
+- The return type / property type must match (EXP0017 if not).
+- Constructor stubs (`[ExpressiveForConstructor]`) must still be `static` methods; instance or property ctor stubs have no coherent meaning.
 
 ## Static Method Mapping
 
@@ -63,10 +69,55 @@ static class EntityMappings
 }
 ```
 
+## Co-located Form (Instance Stub + Single-argument Attribute)
+
+When the target is on the same type as the stub, the most ergonomic form combines an **instance stub** with the **single-argument** attribute. `this` is the receiver automatically. Use this form when a property has its own backing storage -- e.g. a plain settable auto-property used for DTO shape, serialization, or in-memory assignment in tests -- but queries should still compute it from other columns.
+
+A **property stub** is often the cleanest choice for this (no parentheses, reads like the target it replaces):
+
+```csharp
+public class Person
+{
+    public string FirstName { get; set; } = "";
+    public string LastName { get; set; } = "";
+
+    // Regular auto-property — assignable directly (for DTOs, tests, deserialization).
+    public string FullName { get; set; } = "";
+
+    // When FullName appears in a LINQ expression tree, it is rewritten to this body,
+    // so EF Core projects it from FirstName/LastName instead of mapping it to its own column.
+    [ExpressiveFor(nameof(FullName))]
+    private string FullNameExpression => FirstName + " " + LastName;
+}
+```
+
+A **method stub** is equivalent in behaviour and appropriate when the target is a method or when you need a block body:
+
+```csharp
+[ExpressiveFor(nameof(FullName))]
+private string FullNameExpression() => FirstName + " " + LastName;
+```
+
+Both forms are equivalent to the verbose `[ExpressiveFor(typeof(Person), nameof(Person.FullName))] static string FullName(Person obj) => obj.FirstName + " " + obj.LastName;` but reuse `this` instead of threading a receiver parameter. When the EF Core integration is enabled, both the target property **and** the stub property itself are automatically excluded from the model (no `[NotMapped]` needed -- see [Automatic NotMapped for `[ExpressiveFor]` targets](#automatic-notmapped-for-expressivefor-targets)).
+
+::: warning When to prefer `[Expressive]` instead
+If the property has no backing storage and the same body works at both runtime and query time, put `[Expressive]` directly on it (`[Expressive] public string FullName => FirstName + " " + LastName;`) and skip the stub. `[ExpressiveFor]` is for the dual-body case; `[Expressive]` is for the single-body case.
+:::
+
 ::: tip
 The stub can use any C# syntax that `[Expressive]` supports -- switch expressions, pattern matching, null-conditional operators, and more.
 :::
 
+## Automatic NotMapped for `[ExpressiveFor]` targets
+
+When `UseExpressives()` is active, EF Core's model builder automatically ignores properties that are:
+
+1. Decorated with `[Expressive]`,
+2. Decorated with `[ExpressiveFor]` (a property stub itself), **or**
+3. The target of an `[ExpressiveFor]` stub anywhere in the loaded assemblies.
+
+You do not need to add `[NotMapped]` to a property you are expressing externally or using as a property stub -- the `ExpressivePropertiesNotMappedConvention` detects these cases via attribute metadata and the generated registry and calls `Ignore()` for you.
+
 ## Constructor Mapping with `[ExpressiveForConstructor]`
 
 Use `[ExpressiveForConstructor]` to provide an expression-tree body for a constructor on a type you do not own:
@@ -114,7 +165,6 @@ The following diagnostics are specific to `[ExpressiveFor]` and `[ExpressiveForC
 |------|----------|-------------|
 | [EXP0014](./diagnostics#exp0014) | Error | Target type specified in `[ExpressiveFor]` could not be resolved |
 | [EXP0015](./diagnostics#exp0015) | Error | No member with the given name found on the target type matching the stub's parameter signature |
-| [EXP0016](./diagnostics#exp0016) | Error | The stub method must be `static` |
 | [EXP0017](./diagnostics#exp0017) | Error | Return type of the stub does not match the target member's return type |
 | [EXP0019](./diagnostics#exp0019) | Error | The target member already has `[Expressive]` -- remove one of the two attributes |
 | [EXP0020](./diagnostics#exp0020) | Error | Duplicate mapping -- only one stub per target member is allowed |

docs/reference/projectable-properties.md

@@ -198,7 +198,7 @@ No `[NotMapped]` annotation or manual `modelBuilder.Ignore(...)` call is require
 
 ## Comparison with `[ExpressiveFor]`
 
-`[ExpressiveFor]` is the verbose alternative -- the formula lives in a separate static stub instead of on the property:
+`[ExpressiveFor]` is the alternative -- the formula lives in a separate stub (static or co-located instance method) instead of on the property:
 
 ```csharp
 public class User

src/ExpressiveSharp.Abstractions/Mapping/ExpressiveForAttribute.cs

@@ -12,13 +12,14 @@ namespace ExpressiveSharp.Mapping;
 /// <para>For <b>instance properties</b>, the stub takes a single parameter (the receiver) and returns the property type.</para>
 /// <para>For <b>static properties</b>, the stub is parameterless and returns the property type.</para>
 /// </remarks>
-[AttributeUsage(AttributeTargets.Method, Inherited = false, AllowMultiple = false)]
+[AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = false)]
 public sealed class ExpressiveForAttribute : Attribute
 {
     /// <summary>
-    /// The type that declares the target member.
+    /// The type that declares the target member, or <c>null</c> when the single-argument constructor
+    /// is used (in which case the target defaults to the stub's containing type at generator time).
     /// </summary>
-    public Type TargetType { get; }
+    public Type? TargetType { get; }
 
     /// <summary>
     /// The name of the target member on <see cref="TargetType"/>.
@@ -43,4 +44,14 @@ public ExpressiveForAttribute(Type targetType, string memberName)
         TargetType = targetType;
         MemberName = memberName;
     }
+
+    /// <summary>
+    /// Shorthand for <c>[ExpressiveFor(typeof(ContainingType), memberName)]</c> —
+    /// use when the target member is on the same type as the stub.
+    /// </summary>
+    public ExpressiveForAttribute(string memberName)
+    {
+        TargetType = null;
+        MemberName = memberName;
+    }
 }

src/ExpressiveSharp.EntityFrameworkCore/Infrastructure/Internal/ExpressiveOptionsExtension.cs

@@ -43,6 +43,10 @@ public ExpressiveOptionsExtension(IReadOnlyList<IExpressivePlugin> plugins, bool
     [SuppressMessage("Usage", "EF1001:Internal EF Core API usage.", Justification = "Required to decorate query compiler")]
     public void ApplyServices(IServiceCollection services)
     {
+        // The expressive resolver is stateless at the instance level (all caches are process-static),
+        // so a singleton lifetime is appropriate and cheap to share across scopes.
+        services.TryAddSingleton<IExpressiveResolver, ExpressiveResolver>();
+
         // Register conventions
         services.AddScoped<IConventionSetPlugin, ExpressiveDbSetDiscoveryConventionPlugin>();
         services.AddScoped<IConventionSetPlugin, ExpressivePropertiesNotMappedConventionPlugin>();

src/ExpressiveSharp.EntityFrameworkCore/Infrastructure/Internal/ExpressivePropertiesNotMappedConvention.cs

@@ -1,15 +1,29 @@
 using System.Reflection;
+using ExpressiveSharp.Mapping;
+using ExpressiveSharp.Services;
 using Microsoft.EntityFrameworkCore.Metadata.Builders;
 using Microsoft.EntityFrameworkCore.Metadata.Conventions;
 
 namespace ExpressiveSharp.EntityFrameworkCore.Infrastructure.Internal;
 
 /// <summary>
-/// Convention that marks properties decorated with <see cref="ExpressiveAttribute"/>
-/// as unmapped in the EF Core model, since they have no backing database column.
+/// Convention that marks properties as unmapped in the EF Core model when they have no backing
+/// database column:
+/// <list type="bullet">
+///   <item>decorated with <see cref="ExpressiveAttribute"/>,</item>
+///   <item>decorated with <see cref="ExpressiveForAttribute"/> (the property is a stub itself), or</item>
+///   <item>the target of an <see cref="ExpressiveForAttribute"/> stub elsewhere in the solution.</item>
+/// </list>
 /// </summary>
 public class ExpressivePropertiesNotMappedConvention : IEntityTypeAddedConvention
 {
+    private readonly IExpressiveResolver _resolver;
+
+    public ExpressivePropertiesNotMappedConvention(IExpressiveResolver resolver)
+    {
+        _resolver = resolver;
+    }
+
     public void ProcessEntityTypeAdded(
         IConventionEntityTypeBuilder entityTypeBuilder,
         IConventionContext<IConventionEntityTypeBuilder> context)
@@ -19,7 +33,9 @@ public void ProcessEntityTypeAdded(
 
         foreach (var property in entityTypeBuilder.Metadata.ClrType.GetRuntimeProperties())
         {
-            if (property.GetCustomAttribute<ExpressiveAttribute>() is not null)
+            if (property.GetCustomAttribute<ExpressiveAttribute>() is not null
+                || property.GetCustomAttribute<ExpressiveForAttribute>() is not null
+                || _resolver.FindExternalExpression(property) is not null)
             {
                 entityTypeBuilder.Ignore(property.Name);
             }

src/ExpressiveSharp.EntityFrameworkCore/Infrastructure/Internal/ExpressivePropertiesNotMappedConventionPlugin.cs

@@ -1,13 +1,21 @@
+using ExpressiveSharp.Services;
 using Microsoft.EntityFrameworkCore.Metadata.Conventions;
 using Microsoft.EntityFrameworkCore.Metadata.Conventions.Infrastructure;
 
 namespace ExpressiveSharp.EntityFrameworkCore.Infrastructure.Internal;
 
 public class ExpressivePropertiesNotMappedConventionPlugin : IConventionSetPlugin
 {
+    private readonly IExpressiveResolver _resolver;
+
+    public ExpressivePropertiesNotMappedConventionPlugin(IExpressiveResolver resolver)
+    {
+        _resolver = resolver;
+    }
+
     public ConventionSet ModifyConventions(ConventionSet conventionSet)
     {
-        conventionSet.EntityTypeAddedConventions.Add(new ExpressivePropertiesNotMappedConvention());
+        conventionSet.EntityTypeAddedConventions.Add(new ExpressivePropertiesNotMappedConvention(_resolver));
         return conventionSet;
     }
 }