Fix for #492

Better handling for null an empty values to SetRemove(Async), Redis will throw here so there's no reason we can't gracefully hamdle it better.

StackExchange.Redis.Tests/Sets.cs

-using System.Linq;
+using System;
+using System.Linq;
+using System.Threading.Tasks;
 using Xunit;
 using Xunit.Abstractions;
 
@@ -35,5 +37,23 @@ public void SScan()
                 }
             }
         }
+
+        [Fact]
+        public async Task SetRemoveArgTests()
+        {
+            using (var conn = Create())
+            {
+                var db = conn.GetDatabase();
+                var key = Me();
+
+                RedisValue[] values = null;
+                Assert.Throws<ArgumentNullException>(() => db.SetRemove(key, values, CommandFlags.HighPriority));
+                await Assert.ThrowsAsync<ArgumentNullException>(async () => await db.SetRemoveAsync(key, values, CommandFlags.HighPriority).ForAwait()).ForAwait();
+
+                values = new RedisValue[0];
+                Assert.Equal(0, db.SetRemove(key, values, CommandFlags.HighPriority));
+                Assert.Equal(0, await db.SetRemoveAsync(key, values, CommandFlags.HighPriority).ForAwait());
+            }
+        }
     }
 }

StackExchange.Redis/StackExchange/Redis/RedisDatabase.cs

@@ -1283,6 +1283,8 @@ public bool SetRemove(RedisKey key, RedisValue value, CommandFlags flags = Comma
 
         public long SetRemove(RedisKey key, RedisValue[] values, CommandFlags flags = CommandFlags.None)
         {
+            if (values == null) throw new ArgumentNullException(nameof(values));
+            if (values.Length == 0) return 0;
             var msg = Message.Create(Database, flags, RedisCommand.SREM, key, values);
             return ExecuteSync(msg, ResultProcessor.Int64);
         }
@@ -1295,6 +1297,8 @@ public Task<bool> SetRemoveAsync(RedisKey key, RedisValue value, CommandFlags fl
 
         public Task<long> SetRemoveAsync(RedisKey key, RedisValue[] values, CommandFlags flags = CommandFlags.None)
         {
+            if (values == null) throw new ArgumentNullException(nameof(values));
+            if (values.Length == 0) return CompletedTask<long>.FromResult(0, asyncState);
             var msg = Message.Create(Database, flags, RedisCommand.SREM, key, values);
             return ExecuteAsync(msg, ResultProcessor.Int64);
         }