public void Should_Check_Replace()
{
//arrange
string str = "hellohe";
string oldVal = "he";
string newVal = "Het";
string resultStr = "HetlloHet";
MyStringBuilder builder = new MyStringBuilder(str);
//act
var result = builder.Replace(oldVal, newVal);
//assert
result.ToString().ShouldBeEquivalentTo(resultStr);
}