Assigning values for Nullable<Guid>

Question

I am trying a create a variable Guid? which represents a nullable db column. I am facing issues while assigning values. I was able to assign null to the variable easily. But when I want to assign a Guid value I have to create a new Guid? object which doesn't accept the string.

Guid? sample=new Guid?(someString); // erroing "Cannot implicitly convert string to Guid"

So I have to do like this

Guid? sample=new Guid?(new Guid(someString)); // working code

I am also assigning values conditionally.

Guid? sample=(string.IsNullOrWhiteSpace(someString))?null: new Guid?(new Guid(someString)); 

Why is this issue coming up with Guid ?

Answer 1

Guid? sample = new Guid?(new Guid(someString));

This does not really make much sense. You have a nullable type to be able to store a null value. But when you are assigning a value anyway, then it won’t be null. A type’s constructor will actually never return null, so new Guid(someString) will always return a valid Guid—or throw an exception of course. As such, you can just assign the resulting Guid to the variable of nullable type:

Guid? sample = new Guid(someString);

If you want to consider invalid input, e.g. if someString is null or simply not a valid Guid, then you can use Guid.TryParse (.NET 4.0 or higher):

Guid? sample = null;
Guid parsed;
if (Guid.TryParse(someString, out parsed))
    sample = parsed;

Note that this will allow you to remove any other check on someString too. You can pass a null string or anything else in, and TryParse will just return false, keeping sample’s default value.

Answer 2

But when I want to assign a Guid value I have to create a new Guid? object which doesn't accept the string.

You can shorten your code to :

Guid? sample= new Guid(someString);

Nullable Types can be assigned the <T> type directly.

int? i = 1; 
int? i2 = null;

bool? b = true;
bool? b2 = null;

Answer 3

Because Guid has a constructor that takes string parameter.But nullable Guid (Guid?) takes only Guid as parameter.