[System.Security.SecuritySafeCritical]  // auto-generated
        unsafe public static String Copy (String str) {
            if (str==null) {
                throw new ArgumentNullException("str");
            }
            Contract.Ensures(Contract.Result<String>() != null);
            Contract.EndContractBlock();
 
            int length = str.Length;
 
            String result = FastAllocateString(length);
 
            fixed(char* dest = &result.m_firstChar)
                fixed(char* src = &str.m_firstChar) {
                     wstrcpy(dest, src, length);
                }
             return result;
        }

        [System.Security.SecurityCritical]  // auto-generated
        [ResourceExposure(ResourceScope.None)]
        [MethodImplAttribute(MethodImplOptions.InternalCall)]
        internal extern static String FastAllocateString(int length);

 

在自己代码中调用

var fastAllocate =
            typeof (string).GetMethods(BindingFlags.NonPublic | BindingFlags.Static)
                .First(x => x.Name == "FastAllocateString");

var newString = (string)fastAllocate.Invoke(null, new object[] {20});

Console.WriteLine(newString.Length); // 20

这样用了反射，性能并不好。