Understanding the StringBuilder Class

As I mentioned earlier in this article, instances of the StringBuilder class—as opposed to the String class—are mutable, or dynamically changeable. StringBuilder is located in the System.Text namespace. As you can see in the table, it does not have nearly as many members as the String class—but it does have enough to get most jobs done. If you have a situation in which you need to perform many string operations—for example, within a large loop—from a performance viewpoint it probably makes sense to use StringBuilder instances instead of String instances.

Table: Key Instance Members of the StringBuilder Class

There are six different overloads of the StringBuilder constructor, designed so that you can create an instance already containing text and—if desired—set the Length and Capacity properties. As you’d suspect, the shortest StringBuilder constructor simply creates an instance without storing any text in it. The listing demonstrates creating a StringBuilder instance on the fly. Next, the Append method is used to store the contents of a TextBox in the StringBuilder. The Length property is used to truncate the StringBuilder to four characters. Finally, the StringBuilder is converted to a just plain vanilla string and displayed in a message box.

Listing: Creating and Truncating a StringBuilder on the Fly

private void btnSB_Click(object sender,
System.EventArgs e) {
System.Text.StringBuilder theSB =
new System.Text.StringBuilder();
theSB.Append (txtSB1.Text);
theSB.Length = 4;
MessageBox.Show (theSB.ToString(), "StringBuilder",
MessageBoxButtons.OK, MessageBoxIcon.Information);
}

If you run the code shown in the listing, first adding some text to the TextBox, you’ll see that the text has been appended to the StringBuilder, which is truncated at four characters.

Let’s do another StringBuilder example. The next listing appends the contents of three TextBoxes into one StringBuilder. The user then enters two characters in a fourth TextBox. All instances of the first character are replaced in the StringBuilder with the second character, and the StringBuilder is then displayed in a multiline TextBox.

Listing: Appending and Replacing in a StringBuilder

private void btnSB_Click(object sender,
System.EventArgs e) {
System.Text.StringBuilder theSB =
new System.Text.StringBuilder();
theSB.Append (txtSB1.Text);
theSB.Append (txtSB2.Text);
theSB.Append (txtSB3.Text);
txtDoIt.Text = "";
string str = txtReplace.Text;
theSB.Replace (str[0],str[1]);
txtDoIt.Text = theSB.ToString();
}

The result of running this code, entering the strings “A nose ”, “is a nose ”, and “is a nose” in the TextBoxes, and replacing the character “n” with “r”, is shown in the figure.

Now that you've mastered the basic building blocks of the C# language, getting a clear notion of how events work in C# would be a very good thing.

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