Question

If I do the following assignement when vm.TempRowKey is null then will the value of newRowKey be null?

var newRowKey = vm.TempRowKey.DotFormatToRowKey();

Also is there a way that I can make the following throw an exception if the dotFormatRowKey does not have the format x.x where x is a number?

public static string DotFormatToRowKey(this string dotFormatRowKey) {
    var splits = dotFormatRowKey.Split('.')
                 .Select(x => String.Format("{0:d2}", Int32.Parse(x)))
                 .ToList();
    return String.Join(String.Empty, splits.ToArray());
}

Answer 1

when vm.TempRowKey is null

Then TempRowKey.DotFormatToRowKey(); will throw a null-reference exception.

throw an exception if the dotFormatRowKey does not have the format x.x where x is a number?

public static string DotFormatToRowKey(this string dotFormatRowKey) 
{
    if (dotFormatRowKey == null)
        throw new ArgumentNullException("dotFormatRowKey");    

    // maybe @"^\d\d?\.\d\d?$" is a beter regex. 
    // accept only 1|2 digits and nothing before|after
    if (! Regex.IsMatch(dotFormatRowKey, @"\d+\.\d+"))  
       throw new ArgumentException("Expected ##.##, was " + dotFormatRowKey);

    var splits = dotFormatRowKey.Split('.')
                 .Select(x => String.Format("{0:d2}", Int32.Parse(x)))
                 .ToList();  // ToList() is never needed

    // ToArray() not needed in Fx >= 4.0
    return String.Join(String.Empty, splits.ToArray()); 
}

Small detail: You are using both ToList() and ToArray() on splits. That is double work and in .NET 4 you don’t need either.