Can I create a generic function that accepts a Nullable<something>?

Question

I'm working on a method that will accept a StreamWriter, either a string or a nullable value, and a length, and write the value to the StreamWriter, padded to the length. If the value is null, I want to write spaces. I want to do something like the following oversimplified example, which is just for demonstration purposes.

public void FixedWrite(StreamWriter writer, string value, 
        int length) {
    if (value == null) value = "";
    value = value.PadRight(length);
    writer.Write(value);
}

public void FixedWrite<T>(StreamWriter writer, T value, 
        int length) where T : Nullable /* won't work of course */ {
    string strVal;
    if (!value.HasValue) strVal = null;
    else strVal = value.Value.ToString();
    FixedWrite(writer, strVal, length);
}

I could have overloads for all the different underlying types (they're all dates, ints, bools and decimals), but I wanted to see if I could get the generic version working. Is there some way of getting T to always be a Nullable<> type, and to access the Nullable<> properties (HasValue and Value)? Or should I just stick with the underlying-type-specific overloads?

This question poses a similar problem, but in my case the values are already nullables and I just want to write out the value if it has one and spaces if it doesn't.

Answer 1

Try something like this:

public void FixedWrite<T>(StreamWriter writer, Nullable<T> value, int length) 
    where T : struct
{
    string strVal;
    if (!value.HasValue) strVal = null;
    else strVal = value.HasValue.ToString();
    FixedWrite(writer, strVal, length);
}

Answer 2

The simple answer is no.

Even tho it is a struct, it's been specifically disallowed in the CLR due to recursion issues that could arise.

Nullable<Nullable<Nullable<etc.>>>()

Answer 3

public void FixedWrite<T>(StreamWriter writer, Nullable<T> value, int length)

Anything wrong with that?

Answer 4

No need to test for HasValue, since Nullable is a struct, you can impose the "new()" constraint:

public void FixedWrite<T>(StreamWriter writer, T value, int length) where T : new()
{
    // just test for NULL
    string strVal = (value==null) ? string.Empty: value.ToString();
    FixedWrite(writer, strVal, length);
}

Now you call it like so (using the "?" notation is the key):

FixedValue<System.DateTime?>(....)