Check if string have uppercase, lowercase and number

How about?

if(myString.Any(char.IsLower) && myString.Any(char.IsUpper) && myString.Any(char.IsDigit))

You can use char's methods with LINQ:

if (myString.Any(char.IsUpper) &&
    myString.Any(char.IsLower) &&
    myString.Any(char.IsDigit))

For the sake of completess, the classic, non-LINQ way to achieve this:

public static bool HasUpperLowerDigit(string text)
{
    bool hasUpper = false; bool hasLower = false; bool hasDigit = false;
    for (int i = 0; i < text.Length && !(hasUpper && hasLower && hasDigit); i++)
    {
        char c = text[i];
        if (!hasUpper) hasUpper = char.IsUpper(c);
        if (!hasLower) hasLower = char.IsLower(c);
        if (!hasDigit) hasDigit = char.IsDigit(c);
    }
    return hasUpper && hasLower && hasDigit;
}

It is more efficient because it loops every character only once whereas the LINQ approaches need three enumerations.

if (myString.Any(ch => char.IsUpper(ch) &&
    myString.Any(ch => char.IsLower(ch) &&
    myString.Any(ch => char.IsDigit(ch))
    { 
      this.hide();
    }
    else
    {
        MessageBox.Show("Error!");
    }