diff --git a/src/mscorlib/src/System/String.cs b/src/mscorlib/src/System/String.cs
index 3d2ac0ff2a7c..ebcff1fe0f4e 100644
--- a/src/mscorlib/src/System/String.cs
+++ b/src/mscorlib/src/System/String.cs
@@ -3174,7 +3174,17 @@ public static String Concat(params Object[] args) {
                     throw new OutOfMemoryException();
                 }
             }
-            return ConcatArray(sArgs, totalLength);
+
+            string result = FastAllocateString(totalLength);
+            int currPos = 0;
+            for (int i = 0; i < sArgs.Length; i++)
+            {
+                Contract.Assert(currPos <= totalLength - sArgs[i].Length, "[String.Concat](currPos <= totalLength - sArgs[i].Length)");
+                FillStringChecked(result, currPos, sArgs[i]);
+                currPos += sArgs[i].Length;
+            }
+
+            return result;
         }
 
         [ComVisible(false)]
@@ -3319,44 +3329,66 @@ public static String Concat(String str0, String str1, String str2, String str3)
             return result;
         }
 
-        [System.Security.SecuritySafeCritical]  // auto-generated
-        private static String ConcatArray(String[] values, int totalLength) {
-            String result =  FastAllocateString(totalLength);
-            int currPos=0;
-
-            for (int i=0; i<values.Length; i++) {
-                Contract.Assert((currPos <= totalLength - values[i].Length), 
-                                "[String.ConcatArray](currPos <= totalLength - values[i].Length)");
-
-                FillStringChecked(result, currPos, values[i]);
-                currPos+=values[i].Length;
-            }
-
-            return result;
-        }
-
         public static String Concat(params String[] values) {
             if (values == null)
                 throw new ArgumentNullException("values");
             Contract.Ensures(Contract.Result<String>() != null);
-            // Spec#: Consider a postcondition saying the length of this string == the sum of each string in array
             Contract.EndContractBlock();
-            int totalLength=0;
 
-            // Always make a copy to prevent changing the array on another thread.
-            String[] internalValues = new String[values.Length];
-            
-            for (int i=0; i<values.Length; i++) {
+            // It's possible that the input values array could be changed concurrently on another
+            // thread, such that we can't trust that each read of values[i] will be equivalent.
+            // Worst case, we can make a defensive copy of the array and use that, but we first
+            // optimistically try the allocation and copies assuming that the array isn't changing,
+            // which represents the 99.999% case, in particular since string.Concat is used for
+            // string concatenation by the languages, with the input array being a params array.
+
+            // Sum the lengths of all input strings
+            long totalLengthLong = 0;
+            for (int i = 0; i < values.Length; i++)
+            {
                 string value = values[i];
-                internalValues[i] = ((value==null)?(String.Empty):(value));
-                totalLength += internalValues[i].Length;
-                // check for overflow
-                if (totalLength < 0) {
-                    throw new OutOfMemoryException();
+                if (value != null)
+                {
+                    totalLengthLong += value.Length;
                 }
             }
-            
-            return ConcatArray(internalValues, totalLength);
+
+            // If it's too long, fail, or if it's empty, return an empty string.
+            if (totalLengthLong > int.MaxValue)
+            {
+                throw new OutOfMemoryException();
+            }
+            int totalLength = (int)totalLengthLong;
+            if (totalLength == 0)
+            {
+                return string.Empty;
+            }
+
+            // Allocate a new string and copy each input string into it
+            string result = FastAllocateString(totalLength);
+            int copiedLength = 0;
+            for (int i = 0; i < values.Length; i++)
+            {
+                string value = values[i];
+                if (!string.IsNullOrEmpty(value))
+                {
+                    int valueLen = value.Length;
+                    if (valueLen > totalLength - copiedLength)
+                    {
+                        copiedLength = -1;
+                        break;
+                    }
+
+                    FillStringChecked(result, copiedLength, value);
+                    copiedLength += valueLen;
+                }
+            }
+
+            // If we copied exactly the right amount, return the new string.  Otherwise,
+            // something changed concurrently to mutate the input array: fall back to
+            // doing the concatenation again, but this time with a defensive copy. This
+            // fall back should be extremely rare.
+            return copiedLength == totalLength ? result : Concat((string[])values.Clone());
         }
         
         [System.Security.SecuritySafeCritical]  // auto-generated