Cache KeySizes array for EC types

Individual KeySizes instances themselves are immutable, so we can re-use instances there.
Arrays are mutable, so continue to return a copy of the array each time.

src/libraries/Common/src/System/Security/Cryptography/ECDiffieHellmanAndroid.cs

@@ -35,11 +35,7 @@ internal ECDiffieHellmanAndroid(SafeEcKeyHandle ecKeyHandle)
                 KeySizeValue = _key.KeySize;
             }
 
-            public override KeySizes[] LegalKeySizes =>
-                new[] {
-                    new KeySizes(minSize: 256, maxSize: 384, skipSize: 128),
-                    new KeySizes(minSize: 521, maxSize: 521, skipSize: 0)
-                };
+            public override KeySizes[] LegalKeySizes => s_defaultKeySizes.CloneKeySizesArray();
 
             protected override void Dispose(bool disposing)
             {

src/libraries/Common/src/System/Security/Cryptography/ECDiffieHellmanCng.cs

@@ -66,17 +66,8 @@ private void ForceSetKeySize(int newKeySize)
             KeySizeValue = newKeySize;
         }
 
-        public override KeySizes[] LegalKeySizes
-        {
-            get
-            {
-                // Return the three sizes that can be explicitly set (for backwards compatibility)
-                return new[] {
-                    new KeySizes(minSize: 256, maxSize: 384, skipSize: 128),
-                    new KeySizes(minSize: 521, maxSize: 521, skipSize: 0),
-                };
-            }
-        }
+        // Return the three sizes that can be explicitly set (for backwards compatibility)
+        public override KeySizes[] LegalKeySizes => s_defaultKeySizes.CloneKeySizesArray();
 
         public override byte[] DeriveKeyFromHash(
             ECDiffieHellmanPublicKey otherPartyPublicKey,

src/libraries/Common/src/System/Security/Cryptography/ECDiffieHellmanOpenSsl.cs

@@ -45,11 +45,7 @@ public ECDiffieHellmanOpenSsl(int keySize)
             _key = new ECOpenSsl(this);
         }
 
-        public override KeySizes[] LegalKeySizes =>
-            new[] {
-                new KeySizes(minSize: 256, maxSize: 384, skipSize: 128),
-                new KeySizes(minSize: 521, maxSize: 521, skipSize: 0)
-            };
+        public override KeySizes[] LegalKeySizes => s_defaultKeySizes.CloneKeySizesArray();
 
         protected override void Dispose(bool disposing)
         {

src/libraries/Common/src/System/Security/Cryptography/ECDiffieHellmanSecurityTransforms.cs

@@ -27,18 +27,8 @@ internal ECDiffieHellmanSecurityTransforms(SafeSecKeyRefHandle publicKey, SafeSe
                 KeySizeValue = _ecc.SetKeyAndGetSize(SecKeyPair.PublicPrivatePair(publicKey, privateKey));
             }
 
-            public override KeySizes[] LegalKeySizes
-            {
-                get
-                {
-                    // Return the three sizes that can be explicitly set (for backwards compatibility)
-                    return new[]
-                    {
-                        new KeySizes(minSize: 256, maxSize: 384, skipSize: 128),
-                        new KeySizes(minSize: 521, maxSize: 521, skipSize: 0),
-                    };
-                }
-            }
+            // Return the three sizes that can be explicitly set (for backwards compatibility)
+            public override KeySizes[] LegalKeySizes => s_defaultKeySizes.CloneKeySizesArray();
 
             public override int KeySize
             {

src/libraries/Common/src/System/Security/Cryptography/ECDsaAndroid.cs

@@ -68,17 +68,8 @@ private void ForceSetKeySize(int newKeySize)
                 KeySizeValue = newKeySize;
             }
 
-            public override KeySizes[] LegalKeySizes
-            {
-                get
-                {
-                    // Return the three sizes that can be explicitly set (for backwards compatibility)
-                    return new[] {
-                        new KeySizes(minSize: 256, maxSize: 384, skipSize: 128),
-                        new KeySizes(minSize: 521, maxSize: 521, skipSize: 0),
-                    };
-                }
-            }
+            // Return the three sizes that can be explicitly set (for backwards compatibility)
+            public override KeySizes[] LegalKeySizes => s_defaultKeySizes.CloneKeySizesArray();
 
             public override byte[] SignHash(byte[] hash)
             {

src/libraries/Common/src/System/Security/Cryptography/ECDsaCng.cs

@@ -83,16 +83,7 @@ private void ForceSetKeySize(int newKeySize)
             KeySizeValue = newKeySize;
         }
 
-        public override KeySizes[] LegalKeySizes
-        {
-            get
-            {
-                // Return the three sizes that can be explicitly set (for backwards compatibility)
-                return new[] {
-                    new KeySizes(minSize: 256, maxSize: 384, skipSize: 128),
-                    new KeySizes(minSize: 521, maxSize: 521, skipSize: 0),
-                };
-            }
-        }
+        // Return the three sizes that can be explicitly set (for backwards compatibility)
+        public override KeySizes[] LegalKeySizes => s_defaultKeySizes.CloneKeySizesArray();
     }
 }

src/libraries/Common/src/System/Security/Cryptography/ECDsaOpenSsl.cs

@@ -78,17 +78,8 @@ private void ForceSetKeySize(int newKeySize)
             KeySizeValue = newKeySize;
         }
 
-        public override KeySizes[] LegalKeySizes
-        {
-            get
-            {
-                // Return the three sizes that can be explicitly set (for backwards compatibility)
-                return new[] {
-                    new KeySizes(minSize: 256, maxSize: 384, skipSize: 128),
-                    new KeySizes(minSize: 521, maxSize: 521, skipSize: 0),
-                };
-            }
-        }
+        // Return the three sizes that can be explicitly set (for backwards compatibility)
+        public override KeySizes[] LegalKeySizes => s_defaultKeySizes.CloneKeySizesArray();
 
         public override byte[] SignHash(byte[] hash)
         {

src/libraries/Common/src/System/Security/Cryptography/ECDsaSecurityTransforms.cs

@@ -28,17 +28,8 @@ internal ECDsaSecurityTransforms(SafeSecKeyRefHandle publicKey, SafeSecKeyRefHan
                 KeySizeValue = _ecc.SetKeyAndGetSize(SecKeyPair.PublicPrivatePair(publicKey, privateKey));
             }
 
-            public override KeySizes[] LegalKeySizes
-            {
-                get
-                {
-                    // Return the three sizes that can be explicitly set (for backwards compatibility)
-                    return new[] {
-                        new KeySizes(minSize: 256, maxSize: 384, skipSize: 128),
-                        new KeySizes(minSize: 521, maxSize: 521, skipSize: 0),
-                    };
-                }
-            }
+            // Return the three sizes that can be explicitly set (for backwards compatibility)
+            public override KeySizes[] LegalKeySizes => s_defaultKeySizes.CloneKeySizesArray();
 
             public override int KeySize
             {

src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/ECAlgorithm.cs

@@ -19,6 +19,12 @@ public abstract class ECAlgorithm : AsymmetricAlgorithm
             // ECDH and ECMQV are not valid in this context.
         };
 
+        private protected static readonly KeySizes[] s_defaultKeySizes =
+        {
+            new KeySizes(minSize: 256, maxSize: 384, skipSize: 128),
+            new KeySizes(minSize: 521, maxSize: 521, skipSize: 0),
+        };
+
         /// <summary>
         /// When overridden in a derived class, exports the named or explicit <see cref="ECParameters" /> for an ECCurve.
         /// If the curve has a name, the Curve property will contain named curve parameters otherwise it will contain explicit parameters.