Is there a "String.Format" that can accept named input parameters instead of index placeholders?

In C# 6 you can use string interpolation:

string name = "Lisa";
int age = 20;
string str = $"Her name is {name} and she's {age} years old";

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As Doug Clutter mentioned in his comment, string interpolation also supports format string. It's possible to change the format by specifying it after a colon. The following example will output a number with a comma and 2 decimal places:

var str = $"Your account balance is {balance:N2}"

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As Bas mentioned in his answer, string interpolation doesn't support template string. Actually, it has no built in support for that. Fortunately it exists in some great libraries.

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SmartFormat.NET for example has support for named placeholder:

Smart.Format("{Name} from {Address.City}, {Address.State}", user)

// The user object should at least be like that 

public class User
{
    public string Name { get; set; }
    public Address Address { get; set; }
}

public class Address
{
    public string City { get; set; }
    public string State { get; set; }
}

It is available on NuGet.

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Mustache is also a great solution. Bas has described its pros well in his answer.

If you don't have C#6 available in your project you can use Linq's .Aggregate():

    var str = "Her name is @name and she's @age years old";

    var parameters = new Dictionary<string, object>();
    parameters.Add("@name", "Lisa");
    parameters.Add("@age", 10);

    str = parameters.Aggregate(str, (current, parameter)=> current.Replace(parameter.Key, parameter.Value.ToString()));

If you want something matching the specific syntax in the question you can put together a pretty simple class based on Aggregate:

public class StringFormatter{

    public string Str {get;set;}

    public Dictionary<string, object> Parameters {get;set;}

    public StringFormatter(string p_str){
        Str = p_str;
        Parameters = new Dictionary<string, object>();
    }

    public void Add(string key, object val){
        Parameters.Add(key, val);
    }

    public override string ToString(){
        return Parameters.Aggregate(Str, (current, parameter)=> current.Replace(parameter.Key, parameter.Value.ToString()));
    }

}

Usable like:

var str = new StringFormatter("Her name is @name and she's @age years old");
str.Add("@name", "Lisa");
str.Add("@age", 10);

Console.WriteLine(str);

Note that this is clean-looking code that's geared to being easy-to-understand over performance.

If you are ok assigning a local variable that contains the data you use to replace the template parameters, you can use the C# 6.0 string interpolation feature.

The basic principle is that you can do fairly advanced string replacement logic based on input data.

Simple example:

string name = "John";
string message = $"Hello, my name is {name}."

Complex example:

List<string> strings = ...
string summary = $"There are {strings.Count} strings. " 
  + $"The average length is {strings.Select(s => s.Length).Average()}"

Drawbacks:

• No support for dynamic templates (e.g. from a resources file)

(Major) advantages:

• It enforces compile time checks on your template replacement.

A nice open source solution that has almost the same syntax, is Mustache. It has two available C# implementations from what I could find - mustache-sharp and Nustache.

I have worked with mustache-sharp and found that it does not have the same power as the string interpolation, but comes close. E.g. you can do the following (stolen from it's github page).

Hello, {{Customer.Name}}
{{#newline}}
{{#newline}}
{{#with Order}}
{{#if LineItems}}
Here is a summary of your previous order:
{{#newline}}
{{#newline}}
{{#each LineItems}}
    {{ProductName}}: {{UnitPrice:C}} x {{Quantity}}
    {{#newline}}
{{/each}}
{{#newline}}
Your total was {{Total:C}}.
{{#else}}
You do not have any recent purchases.
{{/if}}
{{/with}}