String/StringBuilder

Here is the way you need to use string and stringbuilder.

              string testStirng = "Hello World";
             testStirng += "! Welcome !";
             
             MessageBox.Show(testStirng);
             StringBuilder testStringBuilder = new StringBuilder();
             testStringBuilder.Append("Hello World");
             testStringBuilder.Append("! Welcome !");
            
             MessageBox.Show(testStringBuilder.ToString());


The output will be same.
But there is a difference in the internal implementation of string and stringbuilder.

Difference Between String and String 

String

1.Its a class used to handle strings.
2.Here concatenation is used to combine two strings.
3.String object is used to concatenate two strings.
4.The first string is combined to the other string by creating a new copy in the memory as a string object, and then the old 
string is deleted
5.That is why  "Strings are immutable".
6. Slower

String Builder

1.This is also the class used to handle strings.
2.Here Append method is used.
3.Here, Stringbuilder object is used.
4.Insertion is done on the existing string.
5.Usage of StringBuilder is more efficient in case large amounts of string manipulations have to be performed.
6. Faster

For more information, refer here http://www.c-sharpcorner.com/Blogs/3778/