Why is String.Format static?

Question

Compare

String.Format("Hello {0}", "World");

with

"Hello {0}".Format("World");

Why did the .Net designers choose a static method over an instance method? What do you think?

Answer 1

I think it's because it's a creator method (not sure if there's a better name). All it does is take what you give it and return a single string object. It doesn't operate on an existing object. If it was non-static, you would need a string to begin with.

Answer 2

Because the Format method has nothing to do with a string's current value. The value of the string isn't used. It takes a string and returns one.

Answer 3

I see nothing wrong with it being static..

The semantics of the static method seem to make a lot more sense to me. Perhaps it is because it is a primitive. Where primitives are used to often, you want to make the utility code for working with them as light as possible.. Also, I think the semantics are a lot better with String.Format over "MyString BLAH BLAH {0}".Format ...

Answer 4

Instance methods are good when you have an object that maintains some state; the process of formatting a string does not affect the string you are operating on (read: does not modify its state), it creates a new string.

With extension methods, you can now have your cake and eat it too (i.e. you can use the latter syntax if it helps you sleep better at night).

Answer 5

I haven't tried it yet but you could make an extension method for what you want. I wouldn't do it, but I think it would work.

Also I find String.Format() more in line with other patterned static methods like Int32.Parse(), long.TryParse(), etc.

You cloud also just use a StringBuilder if you want a non static format. StringBuilder.AppendFormat()

Answer 6

String.Format takes at least one String and returns a different String. It doesn't need to modify the format string in order to return another string, so it makes little sense to do that (ignoring your formatting of it). On the other hand, it wouldn't be that much of a stretch to make String.Format be a member function, except I don't think C# allows for const member functions like C++ does. [Please correct me and this post if it does.]

MSN

Answer 7

I think it looks better in general to use String.Format, but I could see a point in wanting to have a non-static function for when you already have a string stored in a variable that you want to "format".

As an aside, all functions of the string class don't act on the string, but return a new string object, because strings are immutable.

Answer 8

Because the Format method has nothing to do with a string's current value.

That's true for all string methods because .NET strings are immutable.

If it was non-static, you would need a string to begin with.

It does: the format string.

I believe this is just another example of the many design flaws in the .NET platform (and I don't mean this as a flame; I still find the .NET framework superior to most other frameworks).

Answer 9

String.Format has to be a static method because strings are immutable. Making it an instance method would imply you could use it to "format" or modify the value of an existing string. This you can't do, and making it an instance method that returned a new string would make no sense. Hence, it's a static method.

Answer 10

First of all thanks for your answers. I really appreciate it.

I tended to accept immutable argument. However Konrad and Kibbee pointed out that nonstatic methods like Trim, Replace, Insert also returning a new instance of string. So i think there must be yet another reason for making Format static.

Answer 11

Well I guess you have to be rather particular about it, but like people are saying, it makes more sense for String.Format to be static because of the implied semantics. Consider:

"Hello {0}".Format("World"); // this makes it sound like Format *modifies* 
                             // the string, which is not possible as 
                             // strings are immutable.

string[] parts = "Hello World".Split(' ');    // this however sounds right, 
                                             // because it implies that you 
                                             // split an existing string into 
                                             // two *new* strings.

Answer 12

Maybe the .NET designers did it this way because JAVA did it this way...

Embrace and extend. :)

See: http://discuss.techinterview.org/default.asp?joel.3.349728.40

Answer 13

The first thing I did when I got to upgrade to VS2008 and C#3, was to do this

public static string F( this string format, params object[] args )
{
    return String.Format(format, args);
}

So I can now change my code from

String.Format("Hello {0}", Name);

to

"Hello {0}".F(Name);

which I find vastly superior.

As for why the .NET designers chose it? Who knows. It's entirely subjective. My money is on either

• Copying Java
• The guy writing it at the time subjectively liked it more.

There aren't really any other valid reasons that I can find

Answer 14

.NET Strings are Immutable

Therefore having an instance method makes absolutely no sense.

String foo = new String();

foo.Format("test {0}",1); // Makes it look like foo should be modified by the Format method. 

string newFoo = String.Format(foo, 1); // Indicates that a new string will be returned, and foo will be unaltered.

Answer 15

Non-overloaded, non-inherited static methods (like Class.b(a,c)) that take an instance as the first variable are semantically equivalent to a method call (like a.b(c)) so the platform team made an arbitrary, aesthetic choice. (Assuming it compiles to the same CIL, which it should.) The only way to know would be to ask them why.

Possibly they did it to keep the two strings close to each other lexigraphically, i.e.

String.Format("Foo {0}", "Bar");

instead of

"Foo {0}".Format("bar");

You want to know what the indexes are mapped to; perhaps they thought that the ".Format" part just adds noise in the middle.

Interestingly, the ToString method (at least for numbers) is the opposite: number.ToString("000") with the format string on the right hand side.

Answer 16

.NET Strings are Immutable
Therefore having an instance method makes absolutely no sense.

By that logic the string class should have no instance methods which return modified copies of the object, yet it has plenty (Trim, ToUpper, and so on). Furthermore, lots of other objects in the framework do this too.

I agree that if they were to make it an instance method, Format seems like it would be a bad name, but that doesn't mean the functionality shouldn't be an instance method.

Why not this? It's consistent with the rest of the .NET framework

"Hello {0}".ToString("Orion");

Answer 17

@Jared:

Non-overloaded, non-inherited static methods (like Class.b(a,c)) that take an instance as the first variable are semantically equivalent to a method call (like a.b(c))

No, they aren't.

(Assuming it compiles to the same CIL, which it should.)

That's your mistake. The CIL produced is different. The distinction is that member methods can't be invoked on null values so the CIL inserts a check against null values. This obviously isn't done in the static variant.

However, String.Format does not allow null values so the developers had to insert a check manually. From this point of view, the member method variant would be technically superior.

Answer 18

I don't actually know the answer but I suspect that it has something to do with the aspect of invoking methods on string literals directly.

If I recall correctly (I didn't actually verify this because I don't have an old IDE handy), early versions of the C# IDE had trouble detecting method calls against string literals in IntelliSense, and that has a big impact on the discoverability of the API. If that was the case, typing the following wouldn't give you any help:

"{0}".Format(12);

If you were forced to type

new String("{0}").Format(12);

It would be clear that there was no advantage to making the Format method an instance method rather than a static method.

The .NET libraries were designed by a lot of the same people that gave us MFC, and the String class in particular bears a strong resemblance to the CString class in MFC. MFC does have an instance Format method (that uses printf style formatting codes rather than the curly-brace style of .NET) which is painful because there's no such thing as a CString literal. So in a MFC codebase that I worked on I see a lot of this:

CString csTemp = "";
csTemp.Format("Some string: %s", szFoo);

which is painful. (I'm not saying that the code above is a great way to do things even in MFC, but that does seem to be the way that most of the developers on the project learned how to use CString::Format). Coming from that heritage, I can imagine that the API designers were trying to avoid that sort of situation again.

Answer 19

This is to avoid confusion with .ToString() methods.

For instance:

double test = 1.54d;

//string.Format pattern
string.Format("This is a test: {0:F1}", test );

//ToString pattern
"This is a test: " + test.ToString("F1");

If Format was an instance method on string this could cause confusion, as the patterns are different.

String.Format() is a utility method to turn multiple objects into a formatted string.

An instance method on a string does something to that string.

Of course, you could do:

public static string FormatInsert( this string input, params object[] args) {
    return string.Format( input, args );
}

"Hello {0}, I have {1} things.".FormatInsert( "world", 3);

Answer 20

I don't know why they did it, but it doesn't really matter anymore:

public static class StringExtension
{
    public static string FormatWith(this string format, params object[] args)
    {
        return String.Format(format, args);
    }
}

public class SomeClass
{
    public string SomeMethod(string name)
    {
        return "Hello, {0}".FormatWith(name);
    }
}

That flows a lot easier, IMHO.

Answer 21

From what i've heard, another reason for String.Format to look it's way is the similarity to function print from C. It was suppose to let C developers have an easier time switching languages.

Answer 22

A big design goal for C# was to make the transition from C/C++ to it as easy as possible. Using dot syntax on a string literal would look very strange to someone with only a C/C++ background, and formatting strings is something a developer will likely do on day one with the language. So I believe they made it static to make it closer to familiar territory.

Answer 23

I think it is because Format doesn't take a string per se, but a "format string". Most strings are equal to things like "Bob Smith" or "1010 Main St" or what have you and not to "Hello {0}", generally you only put those format strings in when you are trying to use a template to create another string, like a factory method, and therefore it lends it self to a static method.