error CS0012: The type 'Expression<>' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.

[Systo01]

I'm trying to dynamically create types, but instead only get some stupid generic errors, although I'm adding all required references.

Version Used:
Visual Studio 2019, 16.9.4
Microsoft.CodeAnalysis.CSharp 3.9.0

Steps to Reproduce:

1. Call MappersCreator.CreateType:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Text;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.Emit;
using DFM = Dapper.FluentMap.Mapping;

namespace Systo.ORM.Dapper
{
    public class MappersCreator
    {
        private Type CreateType(Type type, string name, Dictionary<string,string> mappings)
        {
            var assemblyName = type.Assembly.FullName;
            var nspace = type.Namespace;
            var sb = new StringBuilder();
            foreach (var kv in mappings) {
                sb.Append($"            Map(m => m.{kv.Key}).ToColumn(\"{kv.Value}\");\n");
            }
            var code = $@"using Dapper.FluentMap.Mapping;

namespace {nspace}
{{
    public class {name}Mapper : EntityMap<{name}>
    {{
        public {name}Mapper()
        {{
{sb.ToString()}
        }}
    }}
}}";
            SyntaxTree syntaxTree = CSharpSyntaxTree.ParseText(code);
            CSharpCompilation compilation = CSharpCompilation.Create(
                assemblyName,
                new[] { syntaxTree },
                new MetadataReference[] {
                    MetadataReference.CreateFromFile(typeof(object).Assembly.Location),
                    MetadataReference.CreateFromFile(type.Assembly.Location),
                    MetadataReference.CreateFromFile(typeof(DFM.EntityMap<Type>).Assembly.Location),
                    MetadataReference.CreateFromFile(typeof(Expression<Type>).Assembly.Location),
                    MetadataReference.CreateFromFile(typeof(Enumerable).Assembly.Location),
                },
                new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary)
            );
            using (var memoryStream = new MemoryStream()) {
                EmitResult result = compilation.Emit(memoryStream);
                if (result.Success) {
                    memoryStream.Seek(0, SeekOrigin.Begin);
                    Assembly assembly = Assembly.Load(memoryStream.ToArray());
                    Type ret = assembly.GetType($"{nspace}.{name}");
                    return ret;
                } else {
                    foreach (var diagnostic in result.Diagnostics) {
                        Console.WriteLine(diagnostic);
                    }
                    return null;
                }
            }
        }
    }
}

Expected Behavior:
The type being compiled

Actual Behavior:
(5,32): error CS0012: The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.
(9,13): error CS0012: The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.
(9,13): error CS0012: The type 'Expression<>' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.
(9,30): error CS0012: The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.
(10,13): error CS0012: The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.
(10,13): error CS0012: The type 'Expression<>' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.
(10,33): error CS0012: The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.

Project file:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Dapper" Version="2.0.78" />
    <PackageReference Include="Dapper.FluentMap" Version="2.0.0" />
    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="3.9.0" />
  </ItemGroup>

  <ItemGroup>
    <Reference Include="System.Linq" />
    <Reference Include="System.Linq.Expressions" />
    <Reference Include="netstandard" />
  </ItemGroup>
</Project>

[sharwell]

... MetadataReference.CreateFromFile(typeof(object).Assembly.Location) ...

typeof(T).Assembly.Location will almost never produce a correct reference location for compilation purposes. Compilations use reference assemblies, while typeof(T).Assembly produces an implementation assembly from a different framework and in a different location. MetadataReference targets should come from a reference assembly resolution approach that matches the behavior of MSBuild and/or NuGet.

⚠️ This example is not intended for use in production/shipping code. ⚠️
https://github.com/dotnet/roslyn-sdk/blob/main/src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/ReferenceAssemblies.cs

[Systo01]

Ok, so what would I have to use instead of these two

typeof(object).Assembly
typeof(Expression<Type>).Assembly

and how would I specify the references to the "type" argument and to DFM.EntityMap?