Nullable type as a generic parameter possible?

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?

Albur

Pls pls make your signature IDataRecord from DbDataRecord.. – Solitaire

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;
}

Allegro

I suggest you use "columnValue == DBNull.Value" instead of the 'is' operator, because its slightly faster =) – Comprehensive

Personal preference, but you can use the short form T? instead of Nullable<T> – Perplex

This is fine for value types, but then I think it won't work at all with reference types (e.g. GetValueOrNull<string>) because C# doesn't seem to like Nullable<(ref type)> like "string?". Robert C Barth & James Jones's solutions, below, seem much better to me if that's your need. – Congius

@Congius - right, hence the "where T : struct", if you want reference types you can create a similar method with "where T: class" – Allegro

@Greg - sure, but then you need a second method, and you can't overload the name. As I say, if you want to handle both val and ref types, I think cleaner solutions are presented on this page. – Congius

@Congius There's a good reason for that - nullable allows "sort-of-null", but it still has value semantics, not reference semantics. Nullable`1 is a special type as far as the runtime is concerned, and a lot of "magic" is involved. – Kedron

Not specifically requested in the question, but this is the best option if you want your method to take a T parameter but need to return the nullable form T?. It does have the limitations @Congius mentions though. – Ildaile

With .NET 5 I get the compiler error CS0266 Cannot implicitly convert type 'T' to 'T?'. An explicit conversion exists (are you missing a cast?). What's wrong here and how can I fix it? – Aeon

What if i want to cover such types: long, int, string? String is not a struct. How to make generic with nullable parameter nevertheless? – Undeceive

Answer 2

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

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

    return default(T);
}

Just use it like this:

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

Hinshelwood

This could be shortened to: return rdr.IsDBNull(index) ? default(T) : (T)rdr[index]; – Fiddlewood

I think this question explicitly wants null, not default(T). – Kaon

@Kaon default(T) will return null for reference types, and 0 for numerical types, making the solution more flexible. – Hinshelwood

I think it's clearer to either call this GetValueOrDefault to clarify that it returns default(T) rather than null. Alternatively, you could have it throw an exception if T is not nullable. – Mccurdy

This method has a lot of advantages, and forces you to think about returning something other than null as well. – Gujral

@Kaon for value types you can explicitly tell that you are looking for nulls: GetValueOrDefault<decimal?> – Speciation

Answer 3

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).

Atencio

This solution does not work, as there is a logical difference between NULL and 0 – Allegro

It works if the type he passes is int?. It will return NULL, just like he wants. If he passes int as the type, it will return 0 since an int can't be NULL. Besides the fact that I tried it and it works perfectly. – Atencio

This is the most correct and flexible answer. However, return default is sufficient (you don't need the (T), the compiler will infer it from the signature return type). – Niello

Answer 4

Multiple generic constraints can't be combined in an OR fashion (less restrictive), only in an AND fashion (more restrictive). Meaning that one method can't handle both scenarios. The generic constraints also cannot be used to make a unique signature for the method, so you'd have to use 2 separate method names.

However, you can use the generic constraints to make sure that the methods are used correctly.

In my case, I specifically wanted null to be returned, and never the default value of any possible value types. GetValueOrDefault = bad. GetValueOrNull = good.

I used the words "Null" and "Nullable" to distinguish between reference types and value types. And here is an example of a couple extension methods I wrote that compliments the FirstOrDefault method in System.Linq.Enumerable class.

    public static TSource FirstOrNull<TSource>(this IEnumerable<TSource> source)
        where TSource: class
    {
        if (source == null) return null;
        var result = source.FirstOrDefault();   // Default for a class is null
        return result;
    }

    public static TSource? FirstOrNullable<TSource>(this IEnumerable<TSource?> source)
        where TSource : struct
    {
        if (source == null) return null;
        var result = source.FirstOrDefault();   // Default for a nullable is null
        return result;
    }

Counterproposal

If you want unified overloaded calls - just use the same names in the signatures: TSource FirstOrNull<TSource>(TSource? source) where TSource : class and TSource FirstOrNull<TSource>(TSource? source) where TSource : struct – Eyelid

Answer 5

Disclaimer: This answer works, but is intended for educational purposes only. :) James Jones' solution is probably the best here and certainly the one I'd go with.

C# 4.0's dynamic keyword makes this even easier, if less safe:

public static dynamic GetNullableValue(this IDataRecord record, string columnName)
{
  var val = reader[columnName];

  return (val == DBNull.Value ? null : val);
}

Now you don't need the explicit type hinting on the RHS:

int? value = myDataReader.GetNullableValue("MyColumnName");

In fact, you don't need it anywhere!

var value = myDataReader.GetNullableValue("MyColumnName");

value will now be an int, or a string, or whatever type was returned from the DB.

The only problem is that this does not prevent you from using non-nullable types on the LHS, in which case you'll get a rather nasty runtime exception like:

Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: Cannot convert null to 'int' because it is a non-nullable value type

As with all code that uses dynamic: caveat coder.

Palladous

I'd avoid dynamic types in this case as they are not necessary. Dynamic types have a significant performance overhead. – Phthalein

Answer 6

I think you want to handle Reference types and struct types. I use it to convert XML Element strings to a more typed type. You can remove the nullAlternative with reflection. The formatprovider is to handle the culture dependent '.' or ',' separator in e.g. decimals or ints and doubles. This may work:

public T GetValueOrNull<T>(string strElementNameToSearchFor, IFormatProvider provider = null ) 
    {
        IFormatProvider theProvider = provider == null ? Provider : provider;
        XElement elm = GetUniqueXElement(strElementNameToSearchFor);

        if (elm == null)
        {
            object o =  Activator.CreateInstance(typeof(T));
            return (T)o; 
        }
        else
        {
            try
            {
                Type type = typeof(T);
                if (type.IsGenericType &&
                type.GetGenericTypeDefinition() == typeof(Nullable<>).GetGenericTypeDefinition())
                {
                    type = Nullable.GetUnderlyingType(type);
                }
                return (T)Convert.ChangeType(elm.Value, type, theProvider); 
            }
            catch (Exception)
            {
                object o = Activator.CreateInstance(typeof(T));
                return (T)o; 
            }
        }
    }

You can use it like this:

iRes = helper.GetValueOrNull<int?>("top_overrun_length");
Assert.AreEqual(100, iRes);



decimal? dRes = helper.GetValueOrNull<decimal?>("top_overrun_bend_degrees");
Assert.AreEqual(new Decimal(10.1), dRes);

String strRes = helper.GetValueOrNull<String>("top_overrun_bend_degrees");
Assert.AreEqual("10.1", strRes);

Spender

Answer 7

Just had to do something incredible similar to this. My code:

public T IsNull<T>(this object value, T nullAlterative)
{
    if(value != DBNull.Value)
    {
        Type type = typeof(T);
        if (type.IsGenericType && 
            type.GetGenericTypeDefinition() == typeof(Nullable<>).GetGenericTypeDefinition())
        {
            type = Nullable.GetUnderlyingType(type);
        }

        return (T)(type.IsEnum ? Enum.ToObject(type, Convert.ToInt32(value)) :
            Convert.ChangeType(value, type));
    }
    else 
        return nullAlternative;
}

Arria

Answer 8

The shorter way :

public static T ValueOrDefault<T>(this DataRow reader, string columnName) => 
        reader.IsNull(columnName) ? default : (T) reader[columnName];

return 0 for int, and null for int?

Oreste

Answer 9

Incase it helps someone - I have used this before and seems to do what I need it to...

public static bool HasValueAndIsNotDefault<T>(this T? v)
    where T : struct
{
    return v.HasValue && !v.Value.Equals(default(T));
}

Newsom

Answer 10

This may be a dead thread, but I tend to use the following:

public static T? GetValueOrNull<T>(this DbDataRecord reader, string columnName)
where T : struct 
{
    return reader[columnName] as T?;
}

Polarize

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

Answer 11

I know this is old, but here is another solution:

public static bool GetValueOrDefault<T>(this SqlDataReader Reader, string ColumnName, out T Result)
{
    try
    {
        object ColumnValue = Reader[ColumnName];

        Result = (ColumnValue!=null && ColumnValue != DBNull.Value) ? (T)ColumnValue : default(T);

        return ColumnValue!=null && ColumnValue != DBNull.Value;
    }
    catch
    {
        // Possibly an invalid cast?
        return false;
    }
}

Now, you don't care if T was value or reference type. Only if the function returns true, you have a reasonable value from the database. Usage:

...
decimal Quantity;
if (rdr.GetValueOrDefault<decimal>("YourColumnName", out Quantity))
{
    // Do something with Quantity
}

This approach is very similar to int.TryParse("123", out MyInt);

Deepdyed

It would be good if you worked on your naming conventions. They lack consistency. In one place there is a variable without a capital then there is one with. The same with parameters to the methods. – Jeffcott

Done and done! Hope code looks better now. Bob's your auntie :) All is skookum – Deepdyed

Answer 12

I just encountered the same problem myself.

... = reader["myYear"] as int?; works and is clean.

It works with any type without an issue. If the result is DBNull, it returns null as the conversion fails.

Cytherea

In fact, you could probably do int v=reader["myYear"]??-1; or some other default instead of -1. However, this might bring up issues if the value is DBNull... – Deepdyed

Answer 13

Here is an extension method I've used for years:

public static T GetValue<T>(this DbDataReader reader, string columnName)
{
    if (reader == null) throw new ArgumentNullException(nameof(reader));
    if (string.IsNullOrWhiteSpace(columnName))
        throw new ArgumentException("Value cannot be null or whitespace.", nameof(columnName));

    // do not swallow exceptions here - let them bubble up to the calling API to be handled and/or logged
    var index = reader.GetOrdinal(columnName);
    if (!reader.IsDBNull(index))
    {
        return (T)reader.GetValue(index);
    }
    return default;
}

Phthalein