Named string formatting in C#

Question

Is there any way to format a string by name rather than position in C#?

In python, I can do something like this example (shamelessly stolen from here):

>>> print '%(language)s has %(#)03d quote types.' % \
      {'language': "Python", "#": 2}
Python has 002 quote types.

Is there any way to do this in C#? Say for instance:

String.Format("{some_variable}: {some_other_variable}", ...);

Being able to do this using a variable name would be nice, but a dictionary is acceptable too.

UPDATE: I ended up doing something like this post, but it's definitely not pretty. I'll try out John Sheehan's approach, but if anybody has any other suggestions in the meantime, feel free to add them. :)

UPDATE 2: John Sheehan's approach works pretty well. Accepting it.

Answer 1

I think the closest you'll get is an indexed format:

String.Format("{0} has {1} quote types.", "C#", "1");

There's also String.Replace(), if you're willing to do it in multiple steps and take it on faith that you won't find your 'variables' anywhere else in the string:

string MyString = "{language} has {n} quote types.";
MyString = MyString.Replace("{language}", "C#").Replace("{n}", "1");

Expanding this to use a List:

List<KeyValuePair<string, string>> replacements = GetFormatDictionary();  
foreach (KeyValuePair<string, string> item in replacements)
{
    MyString = MyString.Replace(item.Key, item.Value);
}

You could do that with a Dictionary<string, string> too by iterating it's .Keys collections, but by using a List<KeyValuePair<string, string>> we can take advantage of the List's .ForEach() method and condense it back to a one-liner:

replacements.ForEach(delegate(KeyValuePair<string,string>) item) { MyString = MyString.Replace(item.Key, item.Value);});

A lambda would be even simpler, but I'm still on .Net 2.0. Also note that the .Replace() performance isn't stellar when used iteratively, since strings in .Net are immutable. Also, this requires the MyString variable be defined in such a way that it's accessible to the delegate, so it's not perfect yet.

Answer 2

I doubt this will be possible. The first thing that comes to mind is how are you going to get access to local variable names?

There might be some clever way using LINQ and Lambda expressions to do this however.

Answer 3

string language = "Python";
int numquotes = 2;
string output = language + " has "+ numquotes + " language types.";

Edit: What I should have said was, "No, I don't believe what you want to do is supported by C#. This is as close as you are going to get."

Answer 4

There is no built-in method for handling this.

Here's one method

string myString = "{foo} is {bar} and {yadi} is {yada}".Inject(o);

Here's another

Status.Text = "{UserName} last logged in at {LastLoginDate}".FormatWith(user);

A third improved method partially based on the two above, from Phil Haack

Answer 5

There doesn't appear to be a way to do this out of the box. Though, it looks feasible to implement your own IFormatProvider that links to an IDictionary for values.

var Stuff = new Dictionary<string, object> {
   { "language", "Python" },
   { "#", 2 }
};
var Formatter = new DictionaryFormatProvider();

// Interpret {0:x} where {0}=IDictionary and "x" is hash key
Console.WriteLine string.Format(Formatter, "{0:language} has {0:#} quote types", Stuff);

Outputs:

Python has 2 quote types

The caveat is that you can't mix FormatProviders, so the fancy text formatting can't be used at the same time.

Answer 6

The framework itself does not provide a way to do this, but you can take a look at this post by Scott Hanselman. Example usage:

Person p = new Person();  
string foo = p.ToString("{Money:C} {LastName}, {ScottName} {BirthDate}");  
Assert.AreEqual("$3.43 Hanselman, {ScottName} 1/22/1974 12:00:00 AM", foo);

This code by James Newton-King is similar and works with sub-properties and indexes,

string foo = "Top result for {Name} was {Results[0].Name}".FormatWith(student));

James's code relies on System.Web.UI.DataBinder to parse the string and requires referencing System.Web, which some people don't like to do in non-web applications.

EDIT: Oh and they work nicely with anonymous types, if you don't have an object with properties ready for it:

string name = ...;
DateTime date = ...;
string foo = "{Name} - {Birthday}".FormatWith(new { Name = name, Birthday = date });

Answer 7

I have an implementation I just posted to my blog here: http://haacked.com/archive/2009/01/04/fun-with-named-formats-string-parsing-and-edge-cases.aspx

It addresses some issues that these other implementations have with brace escaping. The post has details. It does the DataBinder.Eval thing too, but is still very fast.

Answer 8

See http://stackoverflow.com/questions/271398#358259

With the linked-to extension you can write this:

var str = "{foo} {bar} {baz}".Format(foo=>"foo", bar=>2, baz=>new object());

and you'll get "foo 2 System.Object".