C# – Nullable type as a generic parameter possible

cgenerics

I want to do something like this :

myYear = record.GetValueOrNull<int?>("myYear"),

Notice the nullable type as the generic parameter.

Since the GetValueOrNull function could return null my first attempt was this: 

public static T GetValueOrNull<T>(this DbDataRecord reader, string columnName)
  where T : class
{
    object columnValue = reader[columnName];

    if (!(columnValue is DBNull))
    {
        return (T)columnValue;
    }
    return null;
}

But the error I'm getting now is:

The type 'int?' must be a reference type in order to use it as parameter 'T' in the generic type or method

Right! Nullable<int> is a struct! So I tried changing the class constraint to a struct constraint (and as a side effect can't return null any more):

public static T GetValueOrNull<T>(this DbDataRecord reader, string columnName)
  where T : struct

Now the assignment:

myYear = record.GetValueOrNull<int?>("myYear");

Gives the following error:

The type 'int?' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method

Is specifying a nullable type as a generic parameter at all possible?

Best Answer

Change the return type to Nullable<T>, and call the method with the non nullable parameter

static void Main(string[] args)
{
    int? i = GetValueOrNull<int>(null, string.Empty);
}


public static Nullable<T> GetValueOrNull<T>(DbDataRecord reader, string columnName) where T : struct
{
    object columnValue = reader[columnName];

    if (!(columnValue is DBNull))
        return (T)columnValue;

    return null;
}
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