а помощь слабо посмотреть?

Public Shared Function Mid
Returns a string containing a specified number of characters from a string.

Parameters
Str
Required. String expression from which characters are returned.
Start
Required. Integer expression. Character position in Str at which the part to be taken starts. If Start is greater than the number of characters in Str, the Mid function returns a zero-length string (""). Start is one based.
Length
Optional. Integer expression. Number of characters to return. If omitted or if there are fewer than Length characters in the text (including the character at position Start), all characters from the start position to the end of the string are returned.

в C# можно использовать String.Substring