# Command-related Questions

## How can I restrict some of my commands so only certain users can execute them?

Based on how you want to implement the restrictions, you can use the 
built-in [RequireUserPermission] precondition, which allows you to 
restrict the command based on the user's current permissions in the 
guild or channel (*e.g. `GuildPermission.Administrator`, 
`ChannelPermission.ManageMessages` etc.*). 

If, however, you wish to restrict the commands based on the user's 
role, you can either create your own custom precondition or use 
Joe4evr's [Preconditions Addons] that provides a few custom 
preconditions that aren't provided in the stock library. 
Its source can also be used as an example for creating your own 
custom preconditions.

[RequireUserPermission]: xref:Discord.Commands.RequireUserPermissionAttribute
[Preconditions Addons]: https://github.com/Joe4evr/Discord.Addons/tree/master/src/Discord.Addons.Preconditions

## I'm getting an error about `Assembly#GetEntryAssembly`. What now?

You may be confusing [CommandService#AddModulesAsync] with 
[CommandService#AddModuleAsync]. The former is used to add modules 
via the assembly, while the latter is used to add a single module.

[CommandService#AddModulesAsync]: xref:Discord.Commands.CommandService#Discord_Commands_CommandService_AddModulesAsync_Assembly_System_IServiceProvider_
[CommandService#AddModuleAsync]: xref:Discord.Commands.CommandService#Discord_Commands_CommandService_AddModuleAsync__1_System_IServiceProvider_

## What does [Remainder] do in the command signature?

The [RemainderAttribute] leaves the string unparsed, meaning you 
don't have to add quotes around the text for the text to be 
recognized as a single object. Please note that if your method has 
multiple parameters, the remainder attribute can only be applied to 
the last parameter.

[!code-csharp[Remainder](samples/commands/Remainder.cs)]

[RemainderAttribute]: xref:Discord.Commands.RemainderAttribute

## What is a service? Why does my module not hold any data after execution?

In Discord.NET, modules are created similarly to ASP.NET, meaning 
that they have a transient nature. This means that they are spawned 
every time when a request is received, and are killed from memory 
when the execution finishes. This is why you cannot store persistent 
data inside a module. To workaround this, consider using a service. 

Service is often used to hold data externally, so that they will 
persist throughout execution. Think of it like a chest that holds 
whatever you throw at it that won't be affected by anything unless 
you want it to. Note that you should also learn Microsoft's 
implementation of [Dependency Injection] before proceeding, as well 
as how it works in [Discord.NET](../guides/commands/commands.md#usage-in-modules).

A brief example of service and dependency injection can be seen below.

[!code-csharp[DI](samples/commands/DI.cs)]

[Dependency Injection]: https://docs.microsoft.com/en-us/aspnet/core/fundamentals/dependency-injection

## I have a long-running Task in my command, and Discord.NET keeps saying that a `MessageReceived` handler is blocking the gateway. What gives?

By default, all commands are executed on the same thread as the 
gateway task, which is responsible for keeping the connection from 
your client to Discord alive. When you execute a long-running task, 
this blocks the gateway from communicating for as long as the command 
task is being executed. The library will warn you about any long 
running event handler (in this case, the command handler) that 
persists for more than 3 seconds. 

To resolve this, the library has designed a flag called [RunMode]. 
There are 2 main `RunMode`s. One being `RunMode.Sync`, which is the 
default; another being `RunMode.Async`. `RunMode.Async` essentially 
calls an unawaited Task and continues with the execution without 
waiting for the command task to finish. You should use 
`RunMode.Async` in either the [CommandAttribute] or the 
[DefaultRunMode] flag in `CommandServiceConfig`. 
Further details regarding `RunMode.Async` can be found below.

[RunMode]: xref:Discord.Commands.RunMode
[CommandAttribute]: xref:Discord.Commands.CommandAttribute
[DefaultRunMode]: xref:Discord.Commands.CommandServiceConfig#Discord_Commands_CommandServiceConfig_DefaultRunMode

## Okay, that's great and all, but how does `RunMode.Async` work, and if it's so great, why is the lib *not* using it by default?

As with any async operation, `RunMode.Async` also comes at a cost. 
The following are the caveats with RunMode.Async,
1) You introduce race condition.
2) Unnecessary overhead caused by [async state machine].
3) [ExecuteAsync] will immediately return [ExecuteResult] instead of 
other result types (this is particularly important for those who wish 
to utilize [RuntimeResult] in 2.0).
4) Exceptions are swallowed.

However, there are ways to remedy #3 and #4.

For #3, in Discord.NET 2.0, the library introduces a new event called 
[CommandExecuted], which is raised whenever the command is 
**successfully executed**. This event will be raised regardless of 
the `RunMode` type and will return the appropriate execution result.

For #4, exceptions are caught in [CommandService#Log] event under 
[LogMessage.Exception] as [CommandException].

[async state machine]: https://www.red-gate.com/simple-talk/dotnet/net-tools/c-async-what-is-it-and-how-does-it-work/))
[ExecuteAsync]: xref:Discord.Commands.CommandService#Discord_Commands_CommandService_ExecuteAsync_Discord_Commands_ICommandContext_System_Int32_System_IServiceProvider_Discord_Commands_MultiMatchHandling_
[ExecuteResult]: xref:Discord.Commands.ExecuteResult
[RuntimeResult]: xref:Discord.Commands.RuntimeResult
[CommandExecuted]: xref:Discord.Commands.CommandService#Discord_Commands_CommandService_CommandExecuted
[CommandService#Log]: xref:Discord.Commands.CommandService#Discord_Commands_CommandService_Log
[LogMessage.Exception]: xref:Discord.LogMessage#Discord_LogMessage_Exception
[CommandException]: xref:Discord.Commands.CommandException