String Interpolation in Visual Studio 2015 and IFormatProvider (CA1305)

You'd use the System.FormattableString or System.IFormattable class:

IFormattable ifs = (IFormattable)$"Hello, {name}";
System.FormattableString fss = $"Hello, {name}";

// pass null to use the format as it was used upon initialization above.
string ifresult = ifs.ToString(null, CultureInfo.InvariantCulture);
string fsresult = fss.ToString(CultureInfo.InvariantCulture);

You need to be compiling against Framework 4.6, as the IFormattable and FormattableString are classes which do not exist in older versions. So if you're targeting older versions of the .NET framework you can't use the interpolation syntax without triggering the error.

Unless you apply a little hack (adapted to compile against 4.6 RTM from Jon Skeet's gist and forked to my own account.). Just add a class file to your project containing:

Update

There is now also a Nuget package available that will provide the same functionality to your project (thanks for bringing this to my attention @habakuk).

install-package StringInterpolationBridge

Or if you want to achieve the same thing without adding an additional assembly to your product add the following code to your project:

namespace System.Runtime.CompilerServices
{
    internal class FormattableStringFactory
    {
        public static FormattableString Create(string messageFormat, params object[] args)
        {
            return new FormattableString(messageFormat, args);
        }
    }
}

namespace System
{
    internal class FormattableString : IFormattable
    {
        private readonly string messageFormat;
        private readonly object[] args;

        public FormattableString(string messageFormat, object[] args)
        {
            this.messageFormat = messageFormat;
            this.args = args;
        }

        public override string ToString()
        {
            return string.Format(messageFormat, args);
        }

        public string ToString(string format, IFormatProvider formatProvider)
        {
            return string.Format(formatProvider, format ?? messageFormat, args);
        }

        public string ToString(IFormatProvider formatProvider)
        {
            return string.Format(formatProvider, messageFormat, args);
        }
    }
}

See:

• https://msdn.microsoft.com/en-us/library/dn961160.aspx

Microsoft has made it easier to use string interpolation and comply with CA1305: Specify IFormatProvider.

If you are using C# 6 or later, you have access to the using static directive.

In addition, the static method FormattableString.Invariant is available for .NET Standard 1.3, .NET Core 1.0 and .NET Framework 4.6 and later. Putting the two together allows you to do this:

using static System.FormattableString;

string name = Invariant($"Hello {name}");

If, however, your goal is for the interpolation to be done via the current culture, then a companion static method FormattableString.CurrentCulture is proposed in .NET Core 3.0 (currently, Preview 5):

using static System.FormattableString;

string name = CurrentCulture($"Hello {name}");

If you're targeting the .NET Framework 4.6, you can take advantage of the fact that string interpolations are implicitly convertible to FormattableString:

From Customizing string interpolation in C# 6 by Thomas Levesque

A lesser known aspect of this feature is that an interpolated string can be treated either as a String, or as an IFormattable, depending on the context.

static string Invariant(FormattableString formattable)
{
    return formattable.ToString(CultureInfo.InvariantCulture);
}

string text = Invariant($"{p.Name} was born on {p.DateOfBirth:D}");