Replace obsolete Precondition sample with something new

docs/guides/commands/preconditions.md

@@ -76,7 +76,7 @@ necessary.
 
 ### Example - Creating a Custom Precondition
 
-[!code-csharp[Custom Precondition](samples/preconditions/require_owner.cs)]
+[!code-csharp[Custom Precondition](samples/preconditions/require_role.cs)]
 
 [CheckPermissionsAsync]: xref:Discord.Commands.PreconditionAttribute.CheckPermissionsAsync*
 [PreconditionResult.FromSuccess]: xref:Discord.Commands.PreconditionResult.FromSuccess*

docs/guides/commands/samples/preconditions/require_role.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Threading.Tasks;
+using Discord.Commands;
+using Discord.WebSocket;
+
+// Inherit from PreconditionAttribute
+public class RequireRoleAttribute : PreconditionAttribute
+{
+    // Create a field to store the specified name
+    private readonly string _name;
+
+    // Create a constructor so the name can be specified
+    public RequireRoleAttribute(string name) => _name = name;
+
+    // Override the CheckPermissions method
+    public override Task<PreconditionResult> CheckPermissionsAsync(ICommandContext context, CommandInfo command, IServiceProvider services)
+    {
+        // Check if this user is a Guild User, which is the only context where roles exist
+        if (context.User is SocketGuildUser gUser)
+        {
+            // If this command was executed by a user with the appropriate role, return a success
+            if (gUser.Roles.Any(r => r.Name == _name))
+                // Since no async work is done, the result has to be wrapped with `Task.FromResult` to avoid compiler errors
+                return Task.FromResult(PreconditionResult.FromSuccess());
+            // Since it wasn't, fail
+            else
+                return Task.FromResult(PreconditionResult.FromError($"You must have a role named {_name} to run this command."));
+        }
+        else
+            return Task.FromResult(PreconditionResult.FromError("You must be in a guild to run this command."));
+    }
+}