Nullable type as a generic parameter possible?

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?

---

将返回类型更改为Nullable，然后使用非空参数调用该方法

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;
}

---

public static T GetValueOrDefault<T>(this IDataRecord rdr, int index)
{
    object val = rdr[index];

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

    return default(T);
}

就像这样使用它：

decimal? Quantity = rdr.GetValueOrDefault<decimal?>(1);
string Unit = rdr.GetValueOrDefault<string>(2);

---

Just do two things to your original code – remove the where constraint, and change the last return from return null to return default(T) . This way you can return whatever type you want.

By the way, you can avoid the use of is by changing your if statement to if (columnValue != DBNull.Value) .

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