Why am I getting an InvalidCastException when using an expression from a static field?

Question

I am just starting to use LinqKit with EntityFramework 6.0.2, and I have the following question ...

Why is this:

public static readonly Expression<Func<MyEnum, string>> ConvertToString = e => 
        e == MyEnum.One
                    ? "one"
                    : e == MyEnum.Two
                        ? "two"
                        : "zero";

private static string GetSomethingElse(IQueryable<EnumTest> things)
{           
    var ret = things
        .AsExpandable()
        .Select(c => Program.ConvertToString.Invoke(c.SomeEnum))
        .First();
    return ret;
}

throw:

An unhandled exception of type 'System.InvalidCastException' 
    occurred in LinqKit.dll

Additional information: Unable to cast object of type     
    'System.Linq.Expressions.FieldExpression' to type 
    'System.Linq.Expressions.LambdaExpression'.

but this:

private static string GetSomething(IQueryable<EnumTest> things)
{
    Expression<Func<MyEnum, string>> ConvertToString = e => e == MyEnum.One
        ? "one"
        : e == MyEnum.Two
            ? "two"
            : "zero";

    var ret = things
        .AsExpandable()
        .Select(c => ConvertToString.Invoke(c.SomeEnum))
        .First();
    return ret;
}

works great?

Answer 1

This is because inside your expression you are accessing the field. An exception indicates that you are accessing a field.

The expression is not evaluated when the request is created. It is executed only after its execution. At this point, he will need to allow the field. The workaround is to get the expression first into a local variable:

private static string GetSomething(IQueryable<EnumTest> things)
{
    var expression = Program.ConvertToString;

    var ret = things
        .AsExpandable()
        .Select(c => expression.Invoke(c.SomeEnum))
        .First();
    return ret;
}

, EntityFramework, , SQL-. , , SQL ( ?). ( ), SQL.