1 files changed, 81 insertions, 0 deletions
diff --git a/src/mscorlib/shared/System/FormattableString.cs b/src/mscorlib/shared/System/FormattableString.cs
new file mode 100644
index 0000000000..6369363b5d
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+++ b/src/mscorlib/shared/System/FormattableString.cs
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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+**
+**
+** Purpose: implementation of the FormattableString
+** class.
+**
+===========================================================*/
+
+namespace System
+{
+    /// <summary>
+    /// A composite format string along with the arguments to be formatted. An instance of this
+    /// type may result from the use of the C# or VB language primitive "interpolated string".
+    /// </summary>
+    public abstract class FormattableString : IFormattable
+    {
+        /// <summary>
+        /// The composite format string.
+        /// </summary>
+        public abstract string Format { get; }
+
+        /// <summary>
+        /// Returns an object array that contains zero or more objects to format. Clients should not
+        /// mutate the contents of the array.
+        /// </summary>
+        public abstract object[] GetArguments();
+
+        /// <summary>
+        /// The number of arguments to be formatted.
+        /// </summary>
+        public abstract int ArgumentCount { get; }
+
+        /// <summary>
+        /// Returns one argument to be formatted from argument position <paramref name="index"/>.
+        /// </summary>
+        public abstract object GetArgument(int index);
+
+        /// <summary>
+        /// Format to a string using the given culture.
+        /// </summary>
+        public abstract string ToString(IFormatProvider formatProvider);
+
+        string IFormattable.ToString(string ignored, IFormatProvider formatProvider)
+        {
+            return ToString(formatProvider);
+        }
+
+        /// <summary>
+        /// Format the given object in the invariant culture. This static method may be
+        /// imported in C# by
+        /// <code>
+        /// using static System.FormattableString;
+        /// </code>.
+        /// Within the scope
+        /// of that import directive an interpolated string may be formatted in the
+        /// invariant culture by writing, for example,
+        /// <code>
+        /// Invariant($"{{ lat = {latitude}; lon = {longitude} }}")
+        /// </code>
+        /// </summary>
+        public static string Invariant(FormattableString formattable)
+        {
+            if (formattable == null)
+            {
+                throw new ArgumentNullException(nameof(formattable));
+            }
+
+            return formattable.ToString(Globalization.CultureInfo.InvariantCulture);
+        }
+
+        public override string ToString()
+        {
+            return ToString(Globalization.CultureInfo.CurrentCulture);
+        }
+    }
+}