From the kind of the question I also get the feeling that the poster wanted to implement something like a tokenizer and his expectation might me along these lines, e.g. if the Val_Str is followed by a whitespace, it is considered as being the first string by it self.
This code would look more like this:
var str = "whatever";
var firstString = "Var_str";
if( str.StartWith(firstString) )
{
if(str.Length == firstString.Length) {
else {
if( Char.IsWhiteSpace(str, firstString.Length)
{
} else {
}
}
else {
}
However this is just a guess on the requirements of the question, so some more details on the expected behavior provided by the poster would be helpful.