Operator '&' cannot be applied to operands of type 'T' and 'T' [duplicate]

Question

My application defines several enums that include the [Flags] attribute.

I wanted to write a small utility method to check if a flag was set for any of those enums and I came up with the following.

protected static bool IsFlagSet<T>(ref T value, ref T flags)
{
    return ((value & flags) == flags);
}

But this gives me the error "Operator '&' cannot be applied to operands of type 'T' and 'T'".

Can this be made to work?

Endow

There is no need for those ref modifiers. – Indulgence

Well, okay. They were used in the example at msdn.microsoft.com/en-us/library/twcad0zb(v=vs.80).aspx. I assumed so they would work efficiently with large value types. – Endow

An enum will never be large enough to make this an optimization. In Swap() they are functional. – Indulgence

Answer 1

& is an operator on a class type. Which means that the class T has to have a method that overloads the operator &.

.Net can't expect that every class will have it. So it fails.

What you can do, is make a base class, that declares the operator overload as a method.

Then use Constraints to declare that T uses that base class:

protected static bool IsFlagSet<T> where T: BaseclassWithAnd (ref T value, ref T flags)
{
    return ((value & flags) == flags);
}

Tonatonal

How do I define an interface with an operator overload?? – Necessarian

i dont think you can define operators in interfaces, only in classes. but i might be wrong – Dunstable

My bad, can't do interface with static method. You could make a base class though. Another option is making a method And() that can be called from the template. – Tonatonal

Enums are not classes, I don't think you can get this to compile/work – Indulgence

For this to work, you'd need to state where T : Enum as the type constraint, but this isn't supported - there's even a specific C# compiler error message for exactly this case. – Goodhumored

Answer 2

The Enum class already has a utility function: Enum.HasFlag(Flag f), see the example on MSDN

 if (petsInFamily.HasFlag(Pet.Dog))
        familiesWithDog++;

Note: This was introduced in C# 4. And while it's very readable it may have some performance issues.

Indulgence

+1 Thanks, I had missed this one and will use it instead of my original approach. However, I feel I should accept an answer that more specifically answers my question. – Endow

+1 - use Enum.HasFlag if you can use .Net 4.0 – Raviv

@Alexei: Ahh, new to .NET 4.0? That explains how I missed it. (Why didn't they add SetFlag() while they were at it?) – Endow

@Alexei: You're right, I looked and it's also listed for Fx 3.5 but that is probably an error. – Indulgence

Note that HasFlag is implemented using Reflection, so consider any possible performance impact. blogs.microsoft.co.il/blogs/bnaya/archive/2011/01/28/… – Pickaback

Answer 3

I understand that my answer is too late, but I found really amazing solution of this problem. Starting from .Net 4 we can use dynamic types in C#. You method can be rewritten:

protected static bool IsFlagSet<T>(T value, T flags)
{
    dynamic a = value;
    dynamic b = flags;
    return ((a & b) == flags);
}

Idea behind that dynamic allows you postpone to runtime if method/operator supported by type T. So if & is defined for T then runtime is success.

Madeline

Answer 4

You must type-cast it to a type that defines the & operation.

    protected static bool IsFlagSet<T>(ref T value, ref T flags)
    {
        return ((Convert.ToInt32(value) & Convert.ToInt32(flags)) == Convert.ToInt32(flags));
    }

Necessarian

Answer 5

The reason of the error is that you can't restric generic type as "have operator X defined for T,T". As result C# has to assume that there is no operator X defined for T,T and shows an error.

This is behavior often discussed in relation to == operator - i.e. Can't operator == be applied to generic types in C#?, but applies to all operators.

For full list of possible constrints see - http://msdn.microsoft.com/en-us/library/d5x73970(v=VS.100).aspx, note that there is no constraint for Enum (that would be useful for your scenario specifically) nor for types with operator X defined.

Raviv