Optimize performance of SecurityIdentifier.ToString()

src/System.Security.Principal.Windows/src/System/Security/Principal/SID.cs

@@ -792,21 +792,44 @@ public override string ToString()
         {
             if (_sddlForm == null)
             {
-                StringBuilder result = new StringBuilder();
-
                 //
-                // Typecasting of _IdentifierAuthority to a long below is important, since
+                // Typecasting of _IdentifierAuthority to a ulong below is important, since
                 // otherwise you would see this: "S-1-NTAuthority-32-544"
                 //
 
-                result.Append("S-1-").Append((long)_identifierAuthority);
-
+#if netcoreapp20 || netstandard
+                StringBuilder result = new StringBuilder();
+                result.Append("S-1-").Append((ulong)_identifierAuthority);
                 for (int i = 0; i < SubAuthorityCount; i++)
                 {
                     result.Append('-').Append((uint)(_subAuthorities[i]));
                 }
-
                 _sddlForm = result.ToString();
+#else
+                // length of buffer calculation
+                // prefix = "S-1-".Length: 4;
+                // authority: ulong.MaxValue.ToString("D") : 20;
+                // subauth = MaxSubAuthorities * ( uint.MaxValue.ToString("D").Length + '-'.Length ): 15 * (10+1): 165;
+                // max possible length = 4 + 20 + 165: 189
+                Span<char> result = stackalloc char[189];
+                result[0] = 'S';
+                result[1] = '-';
+                result[2] = '1';
+                result[3] = '-';
+                int written;
+                int length = 4;
+                ((ulong)_identifierAuthority).TryFormat(result.Slice(length), out written);
+                length += written;
+                int[] values = _subAuthorities;
+                for (int index = 0; index < values.Length; index++)
+                {
+                    result[length] = '-';
+                    length += 1;
+                    ((uint)values[index]).TryFormat(result.Slice(length), out written);
+                    length += written;
+                }
+                _sddlForm = result.Slice(0, length).ToString();
+#endif
             }
 
             return _sddlForm;
@@ -910,9 +933,9 @@ public override IdentityReference Translate(Type targetType)
             }
         }
 
-        #endregion
+#endregion
 
-        #region Operators
+#region Operators
 
         public static bool operator ==(SecurityIdentifier left, SecurityIdentifier right)
         {
@@ -938,9 +961,9 @@ public override IdentityReference Translate(Type targetType)
             return !(left == right);
         }
 
-        #endregion
+#endregion
 
-        #region IComparable implementation
+#region IComparable implementation
 
         public int CompareTo(SecurityIdentifier sid)
         {
@@ -982,9 +1005,9 @@ public int CompareTo(SecurityIdentifier sid)
             return 0;
         }
 
-        #endregion
+#endregion
 
-        #region Public Methods
+#region Public Methods
 
         internal int GetSubAuthority(int index)
         {
@@ -1220,6 +1243,6 @@ internal static IdentityReferenceCollection Translate(IdentityReferenceCollectio
 
             throw new ArgumentException(SR.IdentityReference_MustBeIdentityReference, nameof(targetType));
         }
-        #endregion
+#endregion
     }
 }