Sample

public string GetUser()
{
    var args = System.Environment.GetCommandLineArgs().ToArgDictionary();
    var user = args.GetValueOrDefault("Username", System.Environment.UserName);
    return user.ToLower();
}

Extension Method

public static class CommandLineExtensions
{
    /// <summary>
    /// Returns the value associated with the specified key, or the default value if the key is null or not matched.
    /// </summary>
    public static string GetValueOrDefault(this Dictionary<string, string> args, string key, string defaultValue)
    {
        if (string.IsNullOrEmpty(key)) return defaultValue;
        key = key.ToLower();
        return args.ContainsKey(key) ? args[key] : defaultValue;
    }

    /// <summary>
    /// Creates a Dictionary&lt;<see cref="string"/>, <see cref="T:string[]"/>&gt; from command line arguments.
    /// </summary>
    public static Dictionary<string, string> ToArgDictionary(this string[] args)
    {
        var resultDict = new Dictionary<string, string>();

        for (var i = 0; i < args.Length; i++)
        {
            var arg = args[i];

            if (arg.Length <= 1) continue;
            if (arg[0] != '-') continue;

            arg = arg.ToLower();
            var parameter = arg.Substring(1);

            //Resolve parameter value
            var value = string.Empty;
            if (parameter.Contains('='))
            {
                var newParameter = parameter.Substring(0, parameter.IndexOf('='));
                value = parameter.Substring(parameter.IndexOf('=') + 1, parameter.Length - (parameter.IndexOf('=') + 1));
                parameter = newParameter;
            }
            else if (i < args.Length - 1 && args[i + 1][0] != '-')
            {
                value = args[++i];
            }

            //Set parameter/value to dict
            if (!resultDict.ContainsKey(parameter))
            {
                resultDict.Add(parameter, value);
            }
            else
            {
                resultDict[parameter] = value;
            }
        }
        return resultDict;
    }
}