Declare Lambda Expression as a class constant field

Question

Why isn't it possible to declare a class constant filed of type Lambda Expression. I want Something like this:

class MyClass
{
   public const Expression<Func<string,bool>> MyExpr = (string s) => s=="Hello!";
}

But I get the compile error: Expression cannot contain anonymous methods or lambda expressions

Sandpaper

Are you sure we see your exact code? Do you use s in another sense elsewhere? – Ullund

@JeppeStigNielsen Yes I'm sure. s is not used anywhere else. Don't you have this problem? Can you declare such a field? – Sandpaper

@Alireza: Yes you can. I just copy/pasted the code and it worked just fine. EDIT: Unless, did you mean you wanted public LambdaExpression... instead of public Expression<...? – Neoarsphenamine

@JeppeStigNielsen Careless me! I have problem when it is going to be declared as const. Updated the question. Thanks for your hint. – Sandpaper

Based on your update, you can't use const, you'd have to use readonly. const requires that it be a compile-time constant; something you could imagine being copy/pasted wherever you use MyExpr. readonly will be resolved at runtime as though you just were referencing a field. – Neoarsphenamine

@ChrisSinclair You are right. I lost the main point of the constants. Thanks so much. – Sandpaper

The only const value of type Expression<Func<string, bool>> you can have, is null. But the error message was confusing. I have not tried to compile the above code myself (yet). – Ullund

Answer 1

This is just a limitation of C# and CLR. Only primitive numeric values, string literals and null can be used as a value of a constant field. Expression trees are represented as a normal graph of objects and can't appear as a constant value.

Hassi

Answer 2

Reproduced. That is a strange error message from the compiler. I would expect instead:

error CS0134: '(field)' is of type '(type)'. A const field of a reference type other than string can only be initialized with null.

The message we do get is misleading. Some C# expressions (I am not talking about .NET expression trees Expression<...>) can clearly contain a lambda expression, but they don't say why this particular expression cannot.

The solution is to make a static readonly field instead:

class MyClass
{
   public static readonly Expression<Func<string, bool>> MyExpr
     = s => s == "Hello!";
}

Only one instance of Expression<> will ever be created, but it is no compile-time constant, there is actually some code that will run once (just) before MyClass is used for the first time.

Ullund

Some C# expressions (I am not talking about .NET expression trees Expression<...>) can clearly contain a lambda expression would you please make an example? – Sandpaper

@Sandpaper The expression filtered = myList.Where(x => x.Status.IsValid) does contain a lambda expression. – Ullund