[c#] Check if Nullable Guid is empty in c#

Quoting from an answer from this question.

Guid is a value type, so a variable of type Guid can't be null to start with.

What then if I see this?

public Nullable<System.Guid> SomeProperty { get; set; }

how should I check if this is null? Like this?

(SomeProperty == null)

or like this?

(SomeProperty == Guid.Empty)

Beginning with C# 7.1, you can use default literal to produce the default value of a type when the compiler can infer the expression type.

Console.Writeline(default(Guid));  
   // ouptut: 00000000-0000-0000-0000-000000000000

Console.WriteLine(default(int));  // output: 0

Console.WriteLine(default(object) is null);  // output: True

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/default

Check Nullable<T>.HasValue

if(!SomeProperty.HasValue ||SomeProperty.Value == Guid.Empty)
{
 //not valid GUID
}
else
{
 //Valid GUID
}

If you want be sure you need to check both

SomeProperty == null || SomeProperty == Guid.Empty

Because it can be null 'Nullable' and it can be an empty GUID something like this {00000000-0000-0000-0000-000000000000}

You should use the HasValue property:

SomeProperty.HasValue

For example:

if (SomeProperty.HasValue)
{
    // Do Something
}
else
{
    // Do Something Else
}

FYI

public Nullable<System.Guid> SomeProperty { get; set; }

is equivalent to:

public System.Guid? SomeProperty { get; set; }

The MSDN Reference: http://msdn.microsoft.com/en-us/library/sksw8094.aspx

Note that HasValue will return true for an empty Guid.

bool validGuid = SomeProperty.HasValue && SomeProperty != Guid.Empty;