Assignment operation that does nothing if variable is null?

if you're concerned about doing this all in a single statement, you're out of luck - C# doesn't have this functionality at the linguistic level and doesn't support operator declaration (as does F#) or overloading of the assignment operator (as does C++). There are, however, a couple of options, if none quite as elegant as you requested.

The if statement, as you mentioned, though it can be written as a single line

if(a == null) a = b;

A helper method which uses a ref parameter

public void AssignIfNull<T>(ref T target, T value)
{
    if(target == null) target = value;
}

// ...

AssignIfNull(ref a, b);

Note the above will not work with a Property, since they cannot be passed as a ref parameter.

EDIT: While the above is similar to Interlocked.CompareExchange, such an alternative returns the original value of the first parameter, so it may prove more of a gotcha than implementing the above method.

Or you could carefully rewrite your initial statement to use a null-coalescing (??) operator in the initial assignment.

Updated

Since C# 8 this is possible with the Null-coalescing assignment operator

a ??= b;

This will only assign b to a if a is null

Original Answer

Although the syntax is verbose

(a is null?()=>a=b:(Action)(()=>{}))();

Let's break it apart

(                           // Expression starts here
    a is null               // If a == null...
        ? () => a = b       // return lambda that assigns a = b
        : (Action) (        // Else return next lambda casted as Action
            () => {}        // Empty lambda that does nothing
        )                   // End cast
)                           // Expression ends here
();                         // Execute it!

Anyway I would just use the one liner if if(a is null) { a = b; }

As you said, an if statement is what you need. There is no conditional operator that doesn't assign when null. An if is most suitable in this case (not everything has to be a oneliner).

The best options:

if(a == null)
{
    a = b;
}

Or:

a = a ?? b;

In fact, I believe that the latter is optimized away as a simple if statement.

Assigning a to itself isn't bad. With object references, it is just an assignment of the memory address. For value types, this is just a little chunk of data.

If a is actually a property setter, check inside the setter the value has changes:

private string a;

public string A
{
    get
    {
        return a;
    }
    set
    {
        if (value != a)
        {
            a = value;
        }
    }
}