Ok, here's a tricky one. Hopefully there is an expression guru here who can spot what I am doing wrong here, cause I am just not getting it.

I am building up expressions that I use to filter queries. To ease that process I have a couple of Expression<Func<T, bool>> extension methods that makes my code cleaner and so far they have been working nicely. I have written tests for all of them except one, which I wrote one for today. And that test fails completely with an ArgumentException with a long stack trace. And I just don't get it. Especially since I have been using that method for a while successfully in my queries!

Anyways, here is the stack trace I get when running the test:

failed: System.ArgumentException : An item with the same key has already been added.
    at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
    at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
    at System.Linq.Expressions.ExpressionCompiler.PrepareInitLocal(ILGenerator gen, ParameterExpression p)
    at System.Linq.Expressions.ExpressionCompiler.GenerateInvoke(ILGenerator gen, InvocationExpression invoke, StackType ask)
    at System.Linq.Expressions.ExpressionCompiler.Generate(ILGenerator gen, Expression node, StackType ask)
    at System.Linq.Expressions.ExpressionCompiler.GenerateBinary(ILGenerator gen, BinaryExpression b, StackType ask)
    at System.Linq.Expressions.ExpressionCompiler.Generate(ILGenerator gen, Expression node, StackType ask)
    at System.Linq.Expressions.ExpressionCompiler.GenerateUnliftedAndAlso(ILGenerator gen, BinaryExpression b)
    at System.Linq.Expressions.ExpressionCompiler.GenerateAndAlso(ILGenerator gen, BinaryExpression b, StackType ask)
    at System.Linq.Expressions.ExpressionCompiler.GenerateBinary(ILGenerator gen, BinaryExpression b, StackType ask)
    at System.Linq.Expressions.ExpressionCompiler.Generate(ILGener
c# exception lambda expression expression-trees
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