How to make String Interpolation work with dictionary

Question

I have a dictionary with key value pair, and I need to use String Interpolation and then map the string according to the named parameters. Please help me.

// code removed for brevity
var rawmessage = "Hello my name is {Name}"; // raw message
var dict= new Dictionary<string, object>();
dict.Add("{Name}", response.Name);

I want to be able to do something like:

// Pseudo code
$"Hello my name is {Key}", dict;

Solution I have found till now:

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

And it works fine, but my manager really wants me to use String Interpolation with Dictionary. I am not sure how to achieve this. Please guide me.

Answer 1

The problem is that string interpolation in C# is just a syntactic sugar wrapper for string.Format. The strings get compiled to the same code. Take this code for example:

public string GetGreeting1(string name)
{
    return string.Format("Hello {0}!", name);
}

public string GetGreeting2(string name)
{
    return $"Hello {name}!";
}

The IL output for both methods is identical:

GetGreeting1:
IL_0000:  ldstr       "Hello {0}!"
IL_0005:  ldarg.1     
IL_0006:  call        System.String.Format
IL_000B:  ret         

GetGreeting2:
IL_0000:  ldstr       "Hello {0}!"
IL_0005:  ldarg.1     
IL_0006:  call        System.String.Format
IL_000B:  ret  

So the solution you have is perfectly valid and one I've used before. The only possible improvement would be if you intend on doing lots of replacements, you may find a performance benefit if you switch to using a StringBuilder. You could probably find a library on Nuget that does it for you, but that might just be overkill for your purpose.

As an extra, I made an extension class for this. With an extra bonus I've also added a method for using an object with any number of parameters that uses reflection to do the replacements:

public static class StringReplacementExtensions
{
    public static string Replace(this string source, Dictionary<string, string> values)
    {
        return values.Aggregate(
            source,
            (current, parameter) => current
                .Replace($"{{{parameter.Key}}}", parameter.Value));
    }

    public static string Replace(this string source, object values)
    {
        var properties = values.GetType().GetProperties();

        foreach (var property in properties)
        {
            source = source.Replace(
                $"{{{property.Name}}}", 
                property.GetValue(values).ToString());
        }

        return source;
    }
}

And use like this:

var source = "Hello {name}!";

//Using Dictionary:
var dict = new Dictionary<string, string> { { "name", "David" } };
var greeting1 = source.Replace(dict);
Console.WriteLine(greeting1);

//Using an anonymous object:
var greeting2 = source.Replace(new { name = "David" });
Console.WriteLine(greeting2);

Answer 2

And it works fine, but my manager really wants me to use String Interpolation with Dictionary. I am not sure how to achieve this.

You can't do that. C# interpolated string is not a templating engine but a compile-time feature, which translates to String.Format calls. That's why you cannot use string interpolation in constant expressions either.

I want to use String Interpolation to map the named parameters (which are present in the dictionary as Keys) to their respective values. But I don't want to use String.Replace. Is there any way out?

Actually there are several templating engines you can use (mainly for logging), so you don't need to reinvent the wheel. You might find Serilog interesting, for example.

See also this thread: What's a good way of doing string templating in .NET?

Answer 3

The above answers are not quite correct in claiming that interpolated strings are simply syntactic sugar for string.Format(), they are capable of just a little bit more. This makes it possible to accomplish what you desire with FormattableString and a custom IFormatProvider/ICustomFormatter, like so:

public class DictionaryFormatter : IFormatProvider, ICustomFormatter
{
    private Dictionary<string, string> _dict;
    public DictionaryFormatter(Dictionary<string, string> dict)
    {
        _dict = dict;
    }
    public string Format(string? format, object? arg, IFormatProvider? formatProvider)
    {
        var key = arg?.ToString() ?? String.Empty;
        return _dict[key];
    }

    public object? GetFormat(Type? formatType)
    {
        if (formatType == typeof(ICustomFormatter))
            return this;
        else
            return null;
    }
}

Usage:

FormattableString x = $"This is an interpolated string with the first argument being: {"One"} and the second being {"Two"}";
var dict = new Dictionary<string, string>() {{"One", "First Value"}, {"Two", "Second Value"}};
var result = x.ToString(new DictionaryFormatter(dict));
Console.WriteLine(x.Format);
Console.WriteLine(string.Join(',', x.GetArguments()));
Console.WriteLine(result);

Output:

"This is an interpolated string with the first argument being {0} and the second being {1}"
"One,Two"
"This is an interpolated string with the first argument being First Value and the second being Second Value"