public int GetLetterCount_ParametersAreValid_ReturnsCharsCount(string str)
 {
     // Act
     return(DoWhileMethods.GetLetterCount(str));
 }
 public void GetLetterCount_StrIsNull_ThrowsException()
 {
     // Act
     Assert.Throws <ArgumentNullException>(() => DoWhileMethods.GetLetterCount(null));
 }