Question

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?

Answer 1

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; }