LinqKit System.InvalidCastException When Invoking method-provided expression on member property

Question

Given a simple parent/child class structure. I want to use linqkit to apply a child lambda expression on the parent. I also want the Lambda expression to be provided by a utility method.

public class Foo
{
    public Bar Bar { get; set; }
}

public class Bar
{
    public string Value { get; set; }
    public static Expression<Func<Bar, bool>> GetLambdaX()
    {
        return c => c.Value == "A";
    }
}
...

Expression<Func<Foo, bool>> lx = c => Bar.GetLambdaX().Invoke(c.Bar);
Console.WriteLine(lx.Expand());

The above code throws

System.InvalidCastException: Unable to cast object of type 
'System.Linq.Expressions.MethodCallExpression' to type 
'System.Linq.Expressions.LambdaExpression'.
at LinqKit.ExpressionExpander.VisitMethodCall(MethodCallExpression m)
at LinqKit.ExpressionVisitor.Visit(Expression exp)
at LinqKit.ExpressionVisitor.VisitLambda(LambdaExpression lambda)
at LinqKit.ExpressionVisitor.Visit(Expression exp)
at LinqKit.Extensions.Expand<TDelegate>(Expression`1 expr)

Answer 1

var lambdaX = Bar.GetLambdaX();

Expression<Func<Foo, bool>> lx = c => lambdaX.Invoke(c.Bar);

This works.

Be careful with writing

Expression<Func<Foo, bool>> lx = ...

Before assignment the compiler processes this line and in your case creates a Method Call Expression (i.e. for calling Bar.GetLambdaX()) that is assigned to lx.

If you use var ... then Bar.GetLambdaX(); is called and assigned, the value of which (which is the lambda expression) is used later in lx = ...

Answer 2

2021 Update: This bug has been fixed as of version 1.1.22.

An issue was opened for this bug in September of 2020, and it was fixed in version 1.1.22 released on January 16th 2021.