diff --git a/src/Microsoft.AspNetCore.Http/Internal/InplaceStringFormatter.cs b/src/Microsoft.AspNetCore.Http/Internal/InplaceStringFormatter.cs
new file mode 100644
index 00000000..044c614b
--- /dev/null
+++ b/src/Microsoft.AspNetCore.Http/Internal/InplaceStringFormatter.cs
@@ -0,0 +1,80 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Runtime.CompilerServices;
+
+namespace Microsoft.AspNetCore.Http.Internal
+{
+    public struct InplaceStringFormatter
+    {
+        private int _length;
+        private int _offset;
+        private bool _writing;
+        private string _value;
+
+        public InplaceStringFormatter(int length) : this()
+        {
+            _length = length;
+        }
+
+        public void AppendLength(string s)
+        {
+            AppendLength(s.Length);
+        }
+        public void AppendLength(char c)
+        {
+            AppendLength(1);
+        }
+
+        public void AppendLength(int length)
+        {
+            if (_writing)
+            {
+                throw new InvalidOperationException("Cannot append lenght after write started.");
+            }
+            _length += length;
+        }
+
+        public unsafe void Append(string s)
+        {
+            EnsureValue(s.Length);
+            fixed (char* value = _value)
+            fixed (char* pDomainToken = s)
+            {
+                Unsafe.CopyBlock(value + _offset, pDomainToken, (uint)s.Length * 2);
+                _offset += s.Length;
+            }
+        }
+        public unsafe void Append(char c)
+        {
+            EnsureValue(1);
+            fixed (char* value = _value)
+            {
+                value[_offset++] = c;
+            }
+        }
+
+        private void EnsureValue(int length)
+        {
+            if (_value == null)
+            {
+                _writing = true;
+                _value = new string('\0', _length);
+            }
+            if (_offset + length > _length)
+            {
+                throw new InvalidOperationException($"Not enough space to write '{length}' characters, only '{_length - _offset}' left.");
+            }
+        }
+
+        public override string ToString()
+        {
+            if (_offset != _length)
+            {
+                throw new InvalidOperationException($"Entire reserved lenght was not used. Length: '{_length}', written '{_offset}'.");
+            }
+            return _value;
+        }
+    }
+}
diff --git a/src/Microsoft.AspNetCore.Http/project.json b/src/Microsoft.AspNetCore.Http/project.json
index 792c5989..80fc5466 100644
--- a/src/Microsoft.AspNetCore.Http/project.json
+++ b/src/Microsoft.AspNetCore.Http/project.json
@@ -29,7 +29,8 @@
       "type": "build"
     },
     "Microsoft.Net.Http.Headers": "1.1.0-*",
-    "System.Buffers": "4.0.0-*"
+    "System.Buffers": "4.0.0-*",
+    "System.Runtime.CompilerServices.Unsafe": "4.0.0"
   },
   "frameworks": {
     "net451": {},
diff --git a/test/Microsoft.AspNetCore.Http.Tests/Internal/InplaceStringFormatterTest.cs b/test/Microsoft.AspNetCore.Http.Tests/Internal/InplaceStringFormatterTest.cs
new file mode 100644
index 00000000..c48409b3
--- /dev/null
+++ b/test/Microsoft.AspNetCore.Http.Tests/Internal/InplaceStringFormatterTest.cs
@@ -0,0 +1,54 @@
+using System;
+using Microsoft.AspNetCore.Http.Internal;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Http.Tests.Internal
+{
+    public class InplaceStringFormatterTest
+    {
+        [Fact]
+        public void ToString_ReturnsStringWithAllAppendedValues()
+        {
+            var s1 = "123";
+            var c1 = '4';
+            var s2 = "56789";
+
+            var formatter = new InplaceStringFormatter();
+            formatter.AppendLength(s1);
+            formatter.AppendLength(c1);
+            formatter.AppendLength(s2);
+            formatter.Append(s1);
+            formatter.Append(c1);
+            formatter.Append(s2);
+            Assert.Equal("123456789", formatter.ToString());
+        }
+
+        [Fact]
+        public void ToString_ThrowsIfNotEnoughWritten()
+        {
+            var formatter = new InplaceStringFormatter(5);
+            formatter.Append("123");
+            var exception = Assert.Throws<InvalidOperationException>(() => formatter.ToString());
+            Assert.Equal(exception.Message, "Entire reserved lenght was not used. Length: '5', written '3'.");
+        }
+
+        [Fact]
+        public void AppendLength_IfAppendWasCalled()
+        {
+            var formatter = new InplaceStringFormatter(3);
+            formatter.Append("123");
+
+            var exception = Assert.Throws<InvalidOperationException>(() => formatter.AppendLength(1));
+            Assert.Equal(exception.Message, "Cannot append lenght after write started.");
+        }
+
+        [Fact]
+        public void Append_ThrowsIfNotEnoughSpace()
+        {
+            var formatter = new InplaceStringFormatter(1);
+
+            var exception = Assert.Throws<InvalidOperationException>(() => formatter.Append("123"));
+            Assert.Equal(exception.Message, "Not enough space to write '3' characters, only '1' left.");
+        }
+    }
+}