Checking for null before ToString()

Update 8 years later (wow!) to cover c# 6's null-conditional operator:

var value = maybeNull?.ToString() ?? String.Empty;

Other approaches:

object defaultValue = "default";
attribs.something = (entry.Properties["something"].Value ?? defaultValue).ToString()

I've also used this, which isn't terribly clever but convenient:

public static string ToSafeString(this object obj)
{
    return (obj ?? string.Empty).ToString();
}

If you are targeting the .NET Framework 3.5, the most elegant solution would be an extension method in my opinion.

public static class ObjectExtensions
{
    public static string NullSafeToString(this object obj)
    {
        return obj != null ? obj.ToString() : String.Empty;
    }
}

Then to use:

attribs.something = entry.Properties["something"].Value.NullSafeToString();

Convert.ToString(entry.Properties["something"].Value);

Can you not do:

attribs.something = entry.Properties["something"].Value as string;

As a variation to RexM's answer:

attribs.something = (entry.Properties["something"].Value ?? attribs.something).ToString()

The only downside would be that the attribs.something would be assigned a value (itself, in this example) even if entry.Properties["something"].Value was null - which could be expensive if the .something property did some other processing and/or this line executes a lot (like in a loop).

To do precisely what you're trying to do a helper method can always be used:

CopyIfNotNull(entry.Properties["something"].Value, out attribs.something);

void CopyIfNotNull(string src, out string dest)
{
  if(src != null)
    dest = src;
}

How about using an auxiliary method like this:

attribs.something = getString(
    entry.Properties["something"].Value, 
    attribs.something);

static String getString(
    Object obj,
    String defaultString)
{
    if (obj == null) return defaultString;
    return obj.ToString();
}

Alternatively, you could use the ?? operator:

attribs.something = 
    (entry.Properties["something"].Value ?? attribs.something).ToString();

(note the redundant ToString() call when the value is null)