How can I default a parameter to Guid.Empty in C#?

Solution

You can use new Guid() instead

public void Problem(Guid optional = new Guid())
{
  // when called without parameters this will be true
  var guidIsEmpty = optional == Guid.Empty;
}

You can also use default(Guid)

default(Guid) also will work exactly as new Guid().

Because Guid is a value type not reference type, so, default(Guid) is not equal to null for example, instead, it's equal to calling default constructor.

Which means that this:

public void Problem(Guid optional = default(Guid))
{
  // when called without parameters this will be true
  var guidIsEmpty = optional == Guid.Empty;
}

It's exactly the same as the original example.

Explanation

Why didn't Guid.Empty work?

The reason you are getting the error is because Empty is defined as:

public static readonly Guid Empty;

So, it is a variable, not a constant (defined as static readonly not as const). Compiler can only have compiler-known values as method parameters default values (not runtime-only-known).

The root cause is that you cannot have a const of any struct, unlike enum for example. If you try it, it will not compile.

The reason once more is that struct is not a primitive type.
For a list of all primitive types in .NET see http://msdn.microsoft.com/en-gb/library/system.typecode.aspx
(note that enum usually inherits int, which is a primitive)

But new Guid() is not a constant too!

I'm not saying it needs a constant. It needs something that can be decided in compile time. Empty is a field, so, it's value is not known in compile time (only at very beginning of run time).

Default parameter value must be known at compile-time, which may be a const value, or something defined using a C# feature that makes value known at compile time, like default(Guid) or new Guid() (which is decided at compile time for structs as you cannot modify the struct constructor in code).

While you can provide default or new easily, you cannot provide a const (because it's not a primitive type or an enum as explained above). So, again, not saying that optional parameter itself needs a constant, but compiler known value.

Guid.Empty is equivalent to new Guid(), which is equivalent to default(Guid). So you can use:

public void Problem(Guid optional = default(Guid))

or

public void Problem(Guid optional = new Guid())

Note that the new Foo() value is only applicable when:

• You're really calling the parameterless constructor
• Foo is a value type

In other words, when the compiler knows it's really just the default value for the type :)

(Interestingly, I'm 99.9% sure it won't call any custom new Foo() constructor you may have created. You can't create such a constructor in a value type in C#, but you can do so in IL.)

You can use the default(Foo) option for any type.