Perf | avoid boxing of SqlGuid in SqlBuffer (dotnet#2306)

Author: wilbit

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/TdsParser.cs

@@ -5504,7 +5504,7 @@ internal static object GetNullSqlValue(SqlBuffer nullVal, SqlMetaDataPriv md, Sq
                     break;
 
                 case SqlDbType.UniqueIdentifier:
-                    nullVal.SqlGuid = SqlGuid.Null;
+                    nullVal.SetToNullOfType(SqlBuffer.StorageType.SqlGuid);
                     break;
 
                 case SqlDbType.Bit:

src/Microsoft.Data.SqlClient/netfx/src/Microsoft/Data/SqlClient/TdsParser.cs

@@ -6310,7 +6310,7 @@ internal static object GetNullSqlValue(
                     break;
 
                 case SqlDbType.UniqueIdentifier:
-                    nullVal.SqlGuid = SqlGuid.Null;
+                    nullVal.SetToNullOfType(SqlBuffer.StorageType.SqlGuid);
                     break;
 
                 case SqlDbType.Bit:

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/Server/ValueUtilsSmi.netfx.cs

@@ -152,10 +152,6 @@ private static void GetNullOutputParameterSmi(SmiMetaData metaData, SqlBuffer ta
                     // special case SqlBinary, 'cause tds parser never sets SqlBuffer to null, just to empty!
                     targetBuffer.SqlBinary = SqlBinary.Null;
                 }
-                else if (SqlBuffer.StorageType.SqlGuid == stype)
-                {
-                    targetBuffer.SqlGuid = SqlGuid.Null;
-                }
                 else
                 {
                     targetBuffer.SetToNullOfType(stype);

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlBuffer.cs

@@ -109,7 +109,7 @@ internal struct Storage
         private bool _isNull;
         private StorageType _type;
         private Storage _value;
-        private object _object;    // String, SqlBinary, SqlCachedBuffer, SqlGuid, SqlString, SqlXml
+        private object _object;    // String, SqlBinary, SqlCachedBuffer, SqlString, SqlXml
 
         internal SqlBuffer()
         {
@@ -815,14 +815,17 @@ internal SqlGuid SqlGuid
                 }
                 else if (StorageType.SqlGuid == _type)
                 {
-                    return IsNull ? SqlGuid.Null : (SqlGuid)_object;
+                    return IsNull ? SqlGuid.Null : new SqlGuid(_value._guid);
                 }
                 return (SqlGuid)SqlValue; // anything else we haven't thought of goes through boxing.
             }
             set
             {
                 Debug.Assert(IsEmpty, "setting value a second time?");
-                _object = value;
+                if (!value.IsNull)
+                {
+                    _value._guid = value.Value;
+                }
                 _type = StorageType.SqlGuid;
                 _isNull = value.IsNull;
             }