Cast a null into something?

Question

I had this interesting discussion today with a colleague. We were debating two pieces of code in C#.

Code Snippet 1:

if(!reader.IsDBNull(2))
{
  long? variable1 = reader.GetInt64(2)
}

Code Snippet 2:

long variable1 = reader.IsDBNull(2) ? (long?) null : reader.GetInt64(2)

Question is: is it a good practice to cast null into a nullable long? Or would you rather use the traditional if statement to avoid casting null to nullable long.

Mccrory

Well the cast is for the Conditional Operator you use, it needs the return type of the branches to be similar and snippet2 is compiler error you need it to be long? variable1 – Whangee

As V4Vendetta said, in your second snippet your lh and rh types are different. – Quartic

In snippet1, variable1 only exists inside the if's scope. – Trumaine

Answer 1

The expressions (type?)null, default(type?) and new Nullable<type>() end up being compiled into the same opcodes:

        long? x = (long?)null;
        long? y = default(long?);
        long? z = new Nullable<long>();

is turned into:

    IL_0001: ldloca.s x
    IL_0003: initobj valuetype [mscorlib]System.Nullable`1<int64>
    IL_0009: ldloca.s y
    IL_000b: initobj valuetype [mscorlib]System.Nullable`1<int64>
    IL_0011: ldloca.s z
    IL_0013: initobj valuetype [mscorlib]System.Nullable`1<int64>

In other words, if you are working with nullable types, you are free to use whichever version you like best. Note however, that you should try to avoid arithmetics with nullable types. If you want to return a nullable value from a conditional expression, both possible results must be nullable if one of them can be null. Any other way could cause an exception in that case.

Keener

For the sake of completeness: The expression null as type? is also compiled into that same IL code. In fact, null need not be casted explicitly if the surrounding expression provides a nullable type context, e.g. condition ? null : value as type?. The null in that expression is also compiled to the new Nullable<type>() IL code. – Keener

Answer 2

Instead of

(long?) null

use

default(long?) 

I would refactor above code like

long? variable1 = reader.IsDBNull(2) ? default(long?) : reader.GetInt64(2)

Carmelocarmen

Why are you using second brackets around long? ? – Kortneykoruna

Answer 3

I prefer not to cast null value (it looks odd to me):

long? variable1 = reader.IsDBNull(2) ? null : (long?)reader.GetInt64(2);

Another options:

long? variable1 = reader.IsDBNull(2) ? default : reader.GetInt64(2); // requires C# 7.1
long? variable1 = reader.IsDBNull(2) ? default(long?) : reader.GetInt64(2);
long? variable1 = reader.IsDBNull(2) ? (long?)null : reader.GetInt64(2);
long? variable1 = reader.IsDBNull(2) ? new Nullable<long>() : reader.GetInt64(2);
long? variable1 = reader.IsDBNull(2) ? new long?() : reader.GetInt64(2);
long? variable1 = reader.IsDBNull(2) ? null : new long?(reader.GetInt64(2));

It's just the matter of taste. I think first option is more readable, than others.

UPDATE: Consider also writing some extension methods to make your code more clear:

public static class DataReaderExtensions
{
    public static long? GetNullableInt64(this IDataReader reader, int index)
    {
        if (reader.IsDBNull(index))
            return null;

        return reader.GetInt64(index);
    }
}

In this case you don't use ternary operator (no casting to nullable), and reading values from reader looks more pretty:

long? variable1 = reader.GetNullableInt64(2);

Kortneykoruna

@lazyberezovsky Coz you cannot convert null to long? – Whangee

@Whangee I fixed this typo before your first comment – Kortneykoruna

@lazyberezovsky you still have it up there ...? null : (long?)reader.GetInt64(2) :( – Whangee

@Whangee up there long? variable1 – Kortneykoruna

@lazyberezovsky Do you have compiler near you , it should give no implicit conversion between <null> and long (you have to cast the null in your expression) Hope you get my point ! ! – Whangee

@Whangee can you show me the place where casting between null and long occurs? – Kortneykoruna

Answer 4

Snippet 2 is worth in my case, as in case of null you're gonna get 0, which is a completely valid value for long

Pinwheel

Answer 5

In C# 7.1 you can use the more concise default literal:

var variable1 = reader.IsDBNull(2) ? default : reader.GetInt64(2);

Arboretum