Extract member list argument parsing to utility method

PluralKit.Bot/CommandSystem/ContextArgumentsExt.cs

@@ -1,5 +1,9 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
+using System.Threading.Tasks;
+
+using PluralKit.Core;
 
 namespace PluralKit.Bot
 {
@@ -55,5 +59,27 @@ namespace PluralKit.Bot
             var flags = ctx.Parameters.Flags();
             return potentialMatches.Any(potentialMatch => flags.Contains(potentialMatch));
         }
+
+        public static async Task<List<PKMember>> ParseMemberList(this Context ctx, SystemId? restrictToSystem)
+        {
+            var members = new List<PKMember>();
+
+            // Loop through all the given arguments
+            while (ctx.HasNext())
+            {
+                // and attempt to match a member 
+                var member = await ctx.MatchMember();
+                if (member == null)
+                    // if we can't, big error. Every member name must be valid.
+                    throw new PKError(ctx.CreateMemberNotFoundError(ctx.PopArgument()));
+
+                if (restrictToSystem != null && member.System != restrictToSystem)
+                    throw Errors.NotOwnMemberError; // TODO: name *which* member?
+                
+                members.Add(member); // Then add to the final output list
+            }
+            
+            return members;
+        }
     }
 }

PluralKit.Bot/Commands/Switch.cs

@@ -23,22 +23,8 @@ namespace PluralKit.Bot
         public async Task SwitchDo(Context ctx)
         {
             ctx.CheckSystem();
-            var members = new List<PKMember>();
 
-            // Loop through all the given arguments
+            var members = await ctx.ParseMemberList(ctx.System.Id);
-            while (ctx.HasNext())
-            {
-                // and attempt to match a member 
-                var member = await ctx.MatchMember();
-                if (member == null)
-                    // if we can't, big error. Every member name must be valid.
-                    throw new PKError(ctx.CreateMemberNotFoundError(ctx.PopArgument()));
-                
-                ctx.CheckOwnMember(member); // Ensure they're in our own system
-                members.Add(member); // Then add to the final output list
-            }
-
-            // Finally, do the actual switch
             await DoSwitchCommand(ctx, members);
         }
         public async Task SwitchOut(Context ctx)