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When to use string.format

I'm playing around with some c# sample code...
At one point int the code I need to use string.format() in order to build a string and have the text value of the variable inserted in place. If I don't do this I get an error.

Later on in the code I am concatonating a variable's value with a string and I'm not forced to use string.forrmat as everything glues together as expected.

I have put the 2 pieces of code in the code window so you can see what I am talking about.
All I can think is that in the first instance I am replacing a placeholder with a variable's value but in the second instance I am simply putting 2 strings (1 a variable's value and 1 a hard coded string) end to end.
Is my thinking correct?

Does string.format look for {0} in the first argument and replace it with the value of the second argument?

string subcategory = listCategories.SelectedItem.Text;       
string query = "<Query><Where><Eq><FieldRef Name='ID' /><Value Type='Text'>{0}</Value></Eq></Where></Query>";
query = string.Format(query, subcategory);

-----

public partial class FullList : System.Web.UI.Page
{
    private const string url = "http://MyServer";
    private Lists Proxy;

    protected void Page_Load(object sender, EventArgs e)
    {
        Proxy = new Lists();
        Proxy.Url = url + "/_vti_bin/lists.asmx";

    }
}

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cyberkiwi

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nmarun

Cyberkiwi,

>>It's a matter of preference whether to use String.Format or concatenate stuff together.

I beg to differ from your opinion. This is not a matter of preference. String concatenation (using '+') should be avoided as much as possible.

Strings are immutable types, meaning once assigned a memory location, they cannot be modified in place. It creates a new string every time you 'append' characters to an existing one. This causes a performance hit in your code and that's the reason to avoid it. Using string.format() IS a better way to go.

http://support.microsoft.com/kb/306822 talks about another option of using a StringBuilder class that pretty much does the same thing as string.format, but I have a read of some articles where people have seen string.format to perform better than the StringBuilder class, but haven't tested it myself.

So all in all - avoid string concatenation and use string.format or StringBuilder class.

Arun

Adam Menkes

Also take a look at String.Concat().

example
String.Concat("myid=", 1);

so you don't have to convert the int to a string.

QPR

ASKER

thanks, you confirmed what string.format() does for me