diff --git a/src/Microsoft.Extensions.Primitives/InplaceStringBuilder.cs b/src/Microsoft.Extensions.Primitives/InplaceStringBuilder.cs
new file mode 100644
index 00000000000..bf1d0cd9baa
--- /dev/null
+++ b/src/Microsoft.Extensions.Primitives/InplaceStringBuilder.cs
@@ -0,0 +1,82 @@
+// 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.Diagnostics;
+using System.Runtime.CompilerServices;
+
+namespace Microsoft.Extensions.Primitives
+{
+    [DebuggerDisplay("Value = {_value}")]
+    public struct InplaceStringBuilder
+    {
+        private int _capacity;
+        private int _offset;
+        private bool _writing;
+        private string _value;
+
+        public InplaceStringBuilder(int capacity) : this()
+        {
+            _capacity = capacity;
+        }
+
+        public int Capacity
+        {
+            get { return _capacity; }
+            set
+            {
+                if (value < 0)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(value));
+                }
+                if (_writing)
+                {
+                    throw new InvalidOperationException("Cannot change capacity after write started.");
+                }
+                _capacity = value;
+            }
+        }
+
+        public unsafe void Append(string s)
+        {
+            EnsureCapacity(s.Length);
+            fixed (char* destination = _value)
+            fixed (char* source = s)
+            {
+                //TODO: https://github.com/aspnet/Common/issues/158
+                Unsafe.CopyBlock(destination + _offset, source, (uint)s.Length * 2);
+                _offset += s.Length;
+            }
+        }
+        public unsafe void Append(char c)
+        {
+            EnsureCapacity(1);
+            fixed (char* destination = _value)
+            {
+                destination[_offset++] = c;
+            }
+        }
+
+        private void EnsureCapacity(int length)
+        {
+            if (_value == null)
+            {
+                _writing = true;
+                _value = new string('\0', _capacity);
+            }
+            if (_offset + length > _capacity)
+            {
+                throw new InvalidOperationException($"Not enough capacity to write '{length}' characters, only '{_capacity - _offset}' left.");
+            }
+        }
+
+        public override string ToString()
+        {
+            if (_offset != _capacity)
+            {
+                throw new InvalidOperationException($"Entire reserved capacity was not used. Capacity: '{_capacity}', written '{_offset}'.");
+            }
+            return _value;
+        }
+    }
+}
diff --git a/src/Microsoft.Extensions.Primitives/project.json b/src/Microsoft.Extensions.Primitives/project.json
index eae12cfe4d7..b7e5d8fb63f 100644
--- a/src/Microsoft.Extensions.Primitives/project.json
+++ b/src/Microsoft.Extensions.Primitives/project.json
@@ -16,13 +16,16 @@
     "nowarn": [
       "CS1591"
     ],
-    "xmlDoc": true
+    "xmlDoc": true,
+    "allowUnsafe": true
   },
   "dependencies": {
     "Microsoft.Extensions.HashCodeCombiner.Sources": {
       "type": "build",
       "version": "1.1.0-*"
-    }
+    },
+    "System.Diagnostics.Debug": "4.0.11",
+    "System.Runtime.CompilerServices.Unsafe": "4.0.0"
   },
   "frameworks": {
     "netstandard1.0": {
diff --git a/test/Microsoft.Extensions.Primitives.Tests/InplaceStringBuilderTest.cs b/test/Microsoft.Extensions.Primitives.Tests/InplaceStringBuilderTest.cs
new file mode 100644
index 00000000000..9c9542e406e
--- /dev/null
+++ b/test/Microsoft.Extensions.Primitives.Tests/InplaceStringBuilderTest.cs
@@ -0,0 +1,52 @@
+using System;
+using Microsoft.Extensions.Primitives;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Http.Tests.Internal
+{
+    public class InplaceStringBuilderTest
+    {
+        [Fact]
+        public void ToString_ReturnsStringWithAllAppendedValues()
+        {
+            var s1 = "123";
+            var c1 = '4';
+            var s2 = "56789";
+
+            var formatter = new InplaceStringBuilder();
+            formatter.Capacity += s1.Length + 1 + s2.Length;
+            formatter.Append(s1);
+            formatter.Append(c1);
+            formatter.Append(s2);
+            Assert.Equal("123456789", formatter.ToString());
+        }
+
+        [Fact]
+        public void Build_ThrowsIfNotEnoughWritten()
+        {
+            var formatter = new InplaceStringBuilder(5);
+            formatter.Append("123");
+            var exception = Assert.Throws<InvalidOperationException>(() => formatter.ToString());
+            Assert.Equal(exception.Message, "Entire reserved capacity was not used. Capacity: '5', written '3'.");
+        }
+
+        [Fact]
+        public void Capacity_ThrowsIfAppendWasCalled()
+        {
+            var formatter = new InplaceStringBuilder(3);
+            formatter.Append("123");
+
+            var exception = Assert.Throws<InvalidOperationException>(() => formatter.Capacity = 5);
+            Assert.Equal(exception.Message, "Cannot change capacity after write started.");
+        }
+
+        [Fact]
+        public void Append_ThrowsIfNotEnoughSpace()
+        {
+            var formatter = new InplaceStringBuilder(1);
+
+            var exception = Assert.Throws<InvalidOperationException>(() => formatter.Append("123"));
+            Assert.Equal(exception.Message, "Not enough capacity to write '3' characters, only '1' left.");
+        }
+    }
+}