How to use nullable built-in variables in LINQ?

Question

I'm very new LINQ, and I've a question regarding it here.

I have this very simple class here for demonstration purposes:

public class CurrencyPair
{
    public string? cultureCode;
    public string? currencyName;

    public CurrencyPair(string iCultureCode, string iCurrencyName)
    {
        cultureCode = iCultureCode;
        currencyName = iCurrencyName;
    }

    public CurrencyPair()
    {
        cultureCode = "";
        currencyName = "";
    }
}

Then I have a List of instances of the above Class:

static List<CurrencyPair> currencyPairs;

Now I'm trying to do this:

public List<string> GetCurrencyNames()
{
    return (from c in currencyPairs select c.currencyName).Distinct().ToList();
}

However I get this error:

The type 'string' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable<T>'

If I remove the ? for cultureCode and currencyName in the class definition, this error goes away.

So how can I use nullable strings in LINQ queries .. ?

Answer 1

string is already a reference type , it can hold null , you don't have to use string?

The error is indicating that as well:

The type 'string' must be a non-nullable value type....

You can only use Nullable<T> with value types.

Nullable<T>

Represents a value type that can be assigned null.

You are trying to declare field with string? which is equal to Nullable<string> , but this can only be done with value types .

In C# and Visual Basic, you mark a value type as nullable by using the ? notation after the value type. For example, int? in C# or Integer? in Visual Basic declares an integer value type that can be assigned null

.