Why isn't there a Guid.IsNullOrEmpty() method

Guid is a value type, so a variable of type Guid can't be null to start with. If you want to know if it's the same as the empty guid, you can just use:

if (guid == Guid.Empty)

For one thing, Guid is not nullable. You could check:

myGuid == default(Guid)

which is equivalent to:

myGuid == Guid.Empty

Here is a simple extension method for a nullable Guid.

/// <summary>
/// Determines if a nullable Guid (Guid?) is null or Guid.Empty
/// </summary>
public static bool IsNullOrEmpty(this Guid? guid)
{
  return (!guid.HasValue || guid.Value == Guid.Empty);
}

UPDATE

If you really wanted to use this everywhere you could write another extension method for a regular Guid. It can never be null, so some people won't like this... but it serves the purpose you are looking for and you don't have to know whether you are working with Guid? or Guid (nice for re-factoring etc.).

/// <summary>
/// Determines if Guid is Guid.Empty
/// </summary>
public static bool IsNullOrEmpty(this Guid guid)
{
  return (guid == Guid.Empty);
}

Now you could use someGuid.IsNullOrEmpty(); in all cases, whether you are using Guid or Guid?.

Like I said, some people will complain about the naming because IsNullOrEmpty() implies that the value could be null (when it can't). If you really wanted to, come up with a different name for the extensions like IsNothing() or IsInsignificant() or whatever :)