"No coercion operator is defined" with dynamically generated selection expression

[samfozz]

I'm trying to generate a selection expression at runtime using System.Linq.Expressions.

This expression selects a navigation property - I am able to generate an expression equivalent to the following with no problems and it works well correctly :

 a => new DocumentTag()
                {
                   new Tag()
                        {
                            Id = a.Tag.Id
                        } 
                }

However, the issue seems to come in when I attempt to add a null check to the generated expression, like this :

 a => new DocumentTag()
                {
                    Tag = a.Tag != null
                        ? new Tag()
                        {
                            Id = a.Tag.Id
                        }
                        : null

                };

This expression when manually coded as above works fine, But when I generate one using Expression.Lambda, I get the "No coercion operator" error executing the expression.

I've created an example of the issue here ( this uses the Microsoft.EntityFramworkCore 2.0.1 / Microsoft.EntityFramworkCore.InMemory / 2.0.1 nuget packages ) - This is a full dotnet core app with the dynamic expression generation being in the MakeDynamicExpression method there.

https://gist.github.com/samfozz/58c799d88b24f6c3f70c5a6dff5b57c1

I've can't seem to find the difference between the generated expression and the hard coded one - they are identical in debug view, and I have not been able to find the difference inspecting the objects in the debugger. The generated expression works correctly when compiled to a Func<TEntity,object>.

Exception message: 

No coercion operator is defined between types 'System.Int32' and 'DynamicExpressionProblem.Tag'.

Stack trace:
   at System.Linq.Expressions.Expression.GetUserDefinedCoercionOrThrow(ExpressionType coercionType, Expression expression, Type convertToType)
   at System.Linq.Expressions.Expression.Convert(Expression expression, Type type, MethodInfo method)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.VisitBinary(BinaryExpression node)
   at System.Linq.Expressions.BinaryExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.VisitConditional(ConditionalExpression node)
   at System.Linq.Expressions.ConditionalExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.VisitMemberAssignment(MemberAssignment node)
   at System.Linq.Expressions.ExpressionVisitor.VisitMemberBinding(MemberBinding node)
   at System.Linq.Expressions.ExpressionVisitor.Visit[T](ReadOnlyCollection`1 nodes, Func`2 elementVisitor)
   at System.Linq.Expressions.ExpressionVisitor.VisitMemberInit(MemberInitExpression node)
   at System.Linq.Expressions.MemberInitExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Remotion.Linq.Clauses.SelectClause.TransformExpressions(Func`2 transformation)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.NavigationRewritingQueryModelVisitor.VisitSelectClause(SelectClause selectClause, QueryModel queryModel)
   at Remotion.Linq.Clauses.SelectClause.Accept(IQueryModelVisitor visitor, QueryModel queryModel)
   at Remotion.Linq.QueryModelVisitorBase.VisitQueryModel(QueryModel queryModel)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.Rewrite(QueryModel queryModel, QueryModel parentQueryModel)
   at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.OptimizeQueryModel(QueryModel queryModel, Boolean asyncQuery)
   at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateQueryExecutor[TResult](QueryModel queryModel)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](Expression query, INodeTypeProvider nodeTypeProvider, IDatabase database, IDiagnosticsLogger`1 logger, Type contextType)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass15_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   at Remotion.Linq.QueryableBase`1.GetEnumerator()
   at System.Collections.Generic.List`1.AddEnumerable(IEnumerable`1 enumerable)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at DynamicExpressionProblem.Program.Main(String[] args) in c:\users\sam.foster\documents\visual studio 2017\Projects\DynamicExpressionProblem\DynamicExpressionProblem\Program.cs:line 126

Steps to reproduce

See here :
https://gist.github.com/samfozz/58c799d88b24f6c3f70c5a6dff5b57c1

Further technical details

EF Core version: 2.0.1
Database Provider: InMemory
Operating system: Windows 10
IDE: Visual Studio 2017

[ajcvickers]

@samfozz Looking in the debugger, the difference in the expression trees is in the type of the null constant used int the conditional. Changing it to type object in the explicitly built tree results in identitical expression trees and it then works with EF query:

var mainAssignment = Expression.Bind(navProp,
    Expression.Condition(
        Expression.MakeBinary(ExpressionType.NotEqual, Expression.MakeMemberAccess(parent, navProp),
            Expression.Constant(null)), // <-- Change is here
        Expression.MemberInit(Expression.New(ctor), subAssignments),
        Expression.Constant(null, navProp.PropertyType)));

Leaving this issue open to discuss in triage if this is something we need to address in the query pipeline.

[samfozz]

@ajcvickers Sorry I didn't spot that myself - Thanks for this!