StringBuilder AppendJoin API

src/System.Private.CoreLib/src/System/Text/StringBuilder.cs

@@ -17,6 +17,8 @@
 using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Globalization;
+using System.Diagnostics.Contracts;
+using System.Collections.Generic;
 
 namespace System.Text
 {
@@ -1050,6 +1052,88 @@ public StringBuilder Append(char[] value)
             return this;
         }
 
+        // Append joined values with a separator between each value.
+        public unsafe StringBuilder AppendJoin<T>(char separator, params T[] values)
+        {
+            // Defer argument validation to the internal function
+            return AppendJoinCore(&separator, 1, values);
+        }
+
+        public unsafe StringBuilder AppendJoin<T>(string separator, params T[] values)
+        {
+            separator = separator ?? string.Empty;
+            fixed (char* pSeparator = separator)
+            {
+                // Defer argument validation to the internal function
+                return AppendJoinCore(pSeparator, separator.Length, values);
+            }
+        }
+
+        public unsafe StringBuilder AppendJoin<T>(char separator, IEnumerable<T> values)
+        {
+            // Defer argument validation to the internal function
+            return AppendJoinCore(&separator, 1, values);
+        }
+
+        public unsafe StringBuilder AppendJoin<T>(string separator, IEnumerable<T> values)
+        {
+            separator = separator ?? string.Empty;
+            fixed (char* pSeparator = separator)
+            {
+                // Defer argument validation to the internal function
+                return AppendJoinCore(pSeparator, separator.Length, values);
+            }
+        }
+
+        private unsafe StringBuilder AppendJoinCore<T>(char* separator, int separatorLength, params T[] values)
+        {
+            if (values == null)
+                throw new ArgumentNullException(nameof(values));
+            Contract.Ensures(Contract.Result<StringBuilder>() != null);
+
+            if (values.Length == 0)
+                return this;
+
+            var value = values[0];
+            if (value != null)
+                Append(value.ToString());
+
+            for (var i = 1; i < values.Length; i++)
+            {
+                Append(separator, separatorLength);
+                value = values[i];
+                if (value != null)
+                    Append(value.ToString());
+            }
+            return this;
+        }
+
+        private unsafe StringBuilder AppendJoinCore<T>(char* separator, int separatorLength, IEnumerable<T> values)
+        {
+            if (values == null)
+                throw new ArgumentNullException(nameof(values));
+            Contract.Ensures(Contract.Result<StringBuilder>() != null);
+
+            using (var en = values.GetEnumerator())
+            {
+                if (!en.MoveNext())
+                    return this;
+
+                var value = en.Current;
+                if (value != null)
+                    Append(value.ToString());
+
+                while (en.MoveNext())
+                {
+                    Append(separator, separatorLength);
+                    value = en.Current;
+                    if (value != null)
+                        Append(value.ToString());
+                }
+            }
+            return this;
+        }
+
         /*====================================Insert====================================
         **
         ==============================================================================*/