feature: Allow setting custom error messages for preconditions (#1124)

* Rebase and use in all in-box preconditions

* Silly git....

* Add configurable 'NotAGuild' message

* Respond to feedback

src/Discord.Net.Commands/Attributes/PreconditionAttribute.cs

@@ -21,6 +21,14 @@ namespace Discord.Commands
         public string Group { get; set; } = null;
 
         /// <summary>
+        /// When overridden in a derived class, uses the supplied string
+        /// as the error message if the precondition doesn't pass.
+        /// Setting this for a class that doesn't override
+        /// this property is a no-op.
+        /// </summary>
+        public virtual string ErrorMessage { get { return null; } set { } }
+
+        /// <summary>
         ///     Checks if the <paramref name="command"/> has the sufficient permission to be executed.
         /// </summary>
         /// <param name="context">The context of the command.</param>

src/Discord.Net.Commands/Attributes/Preconditions/RequireBotPermissionAttribute.cs

@@ -17,6 +17,8 @@ namespace Discord.Commands
         ///     Gets the specified <see cref="Discord.ChannelPermission" /> of the precondition.
         /// </summary>
         public ChannelPermission? ChannelPermission { get; }
+        public override string ErrorMessage { get; set; }
+        public string NotAGuildErrorMessage { get; set; }
 
         /// <summary>
         ///     Requires the bot account to have a specific <see cref="Discord.GuildPermission"/>.
@@ -56,9 +58,9 @@ namespace Discord.Commands
             if (GuildPermission.HasValue)
             {
                 if (guildUser == null)
-                    return PreconditionResult.FromError("Command must be used in a guild channel.");
+                    return PreconditionResult.FromError(NotAGuildErrorMessage ?? "Command must be used in a guild channel.");
                 if (!guildUser.GuildPermissions.Has(GuildPermission.Value))
-                    return PreconditionResult.FromError($"Bot requires guild permission {GuildPermission.Value}.");
+                    return PreconditionResult.FromError(ErrorMessage ?? $"Bot requires guild permission {GuildPermission.Value}.");
             }
 
             if (ChannelPermission.HasValue)
@@ -70,7 +72,7 @@ namespace Discord.Commands
                     perms = ChannelPermissions.All(context.Channel);
 
                 if (!perms.Has(ChannelPermission.Value))
-                    return PreconditionResult.FromError($"Bot requires channel permission {ChannelPermission.Value}.");
+                    return PreconditionResult.FromError(ErrorMessage ?? $"Bot requires channel permission {ChannelPermission.Value}.");
             }
 
             return PreconditionResult.FromSuccess();

src/Discord.Net.Commands/Attributes/Preconditions/RequireContextAttribute.cs

@@ -33,6 +33,7 @@ namespace Discord.Commands
         ///     Gets the context required to execute the command.
         /// </summary>
         public ContextType Contexts { get; }
+        public override string ErrorMessage { get; set; }
 
         /// <summary> Requires the command to be invoked in the specified context. </summary>
         /// <param name="contexts">The type of context the command can be invoked in. Multiple contexts can be specified by ORing the contexts together.</param>
@@ -66,7 +67,7 @@ namespace Discord.Commands
             if (isValid)
                 return Task.FromResult(PreconditionResult.FromSuccess());
             else
-                return Task.FromResult(PreconditionResult.FromError($"Invalid context for command; accepted contexts: {Contexts}."));
+                return Task.FromResult(PreconditionResult.FromError(ErrorMessage ?? $"Invalid context for command; accepted contexts: {Contexts}."));
         }
     }
 }

src/Discord.Net.Commands/Attributes/Preconditions/RequireNsfwAttribute.cs

@@ -30,13 +30,15 @@ namespace Discord.Commands
     [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
     public class RequireNsfwAttribute : PreconditionAttribute
     {
+        public override string ErrorMessage { get; set; }
+
         /// <inheritdoc />
         public override Task<PreconditionResult> CheckPermissionsAsync(ICommandContext context, CommandInfo command, IServiceProvider services)
         {
             if (context.Channel is ITextChannel text && text.IsNsfw)
                 return Task.FromResult(PreconditionResult.FromSuccess());
             else
-                return Task.FromResult(PreconditionResult.FromError("This command may only be invoked in an NSFW channel."));
+                return Task.FromResult(PreconditionResult.FromError(ErrorMessage ?? "This command may only be invoked in an NSFW channel."));
         }
     }
 }

src/Discord.Net.Commands/Attributes/Preconditions/RequireOwnerAttribute.cs

@@ -34,6 +34,8 @@ namespace Discord.Commands
     [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
     public class RequireOwnerAttribute : PreconditionAttribute
     {
+        public override string ErrorMessage { get; set; }
+
         /// <inheritdoc />
         public override async Task<PreconditionResult> CheckPermissionsAsync(ICommandContext context, CommandInfo command, IServiceProvider services)
         {
@@ -42,10 +44,10 @@ namespace Discord.Commands
                 case TokenType.Bot:
                     var application = await context.Client.GetApplicationInfoAsync().ConfigureAwait(false);
                     if (context.User.Id != application.Owner.Id)
-                        return PreconditionResult.FromError("Command can only be run by the owner of the bot.");
+                        return PreconditionResult.FromError(ErrorMessage ?? "Command can only be run by the owner of the bot.");
                     return PreconditionResult.FromSuccess();
                 default:
-                    return PreconditionResult.FromError($"{nameof(RequireOwnerAttribute)} is not supported by this {nameof(TokenType)}.");                    
+                    return PreconditionResult.FromError($"{nameof(RequireOwnerAttribute)} is not supported by this {nameof(TokenType)}.");
             }
         }
     }

src/Discord.Net.Commands/Attributes/Preconditions/RequireUserPermissionAttribute.cs

@@ -17,6 +17,8 @@ namespace Discord.Commands
         ///     Gets the specified <see cref="Discord.ChannelPermission" /> of the precondition.
         /// </summary>
         public ChannelPermission? ChannelPermission { get; }
+        public override string ErrorMessage { get; set; }
+        public string NotAGuildErrorMessage { get; set; }
 
         /// <summary>
         ///     Requires that the user invoking the command to have a specific <see cref="Discord.GuildPermission"/>.
@@ -54,9 +56,9 @@ namespace Discord.Commands
             if (GuildPermission.HasValue)
             {
                 if (guildUser == null)
-                    return Task.FromResult(PreconditionResult.FromError("Command must be used in a guild channel."));                
+                    return Task.FromResult(PreconditionResult.FromError(NotAGuildErrorMessage ?? "Command must be used in a guild channel."));                
                 if (!guildUser.GuildPermissions.Has(GuildPermission.Value))
-                    return Task.FromResult(PreconditionResult.FromError($"User requires guild permission {GuildPermission.Value}."));
+                    return Task.FromResult(PreconditionResult.FromError(ErrorMessage ?? $"User requires guild permission {GuildPermission.Value}."));
             }
 
             if (ChannelPermission.HasValue)
@@ -68,7 +70,7 @@ namespace Discord.Commands
                     perms = ChannelPermissions.All(context.Channel);
 
                 if (!perms.Has(ChannelPermission.Value))
-                    return Task.FromResult(PreconditionResult.FromError($"User requires channel permission {ChannelPermission.Value}."));
+                    return Task.FromResult(PreconditionResult.FromError(ErrorMessage ?? $"User requires channel permission {ChannelPermission.Value}."));
             }
 
             return Task.FromResult(PreconditionResult.FromSuccess());