using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        var output = new StringBuilder();

        output.AppendIfNotNull( 0, v => string.Format( "I got added with value {0}!", v ) );

        output.AppendIfNotNull<object>( null, v => "I didn't get added" );

        output.AppendIfNotNull( string.Empty, v => "I did get added" );

        Console.WriteLine( output.ToString() );

    }

}

​

public static class StringBuilderExtension

{

    public static void AppendIfNotNull<TValue>(this StringBuilder sb, TValue value, Func<TValue, string> transform)

    {

        if (value != null)

        {

            sb.AppendLine( transform( value ));

        }

    }

}

​