a9fb8ac1c3
First on constant variable issue:
This will not compile:
[Plugin(MyPluginName)] // Won't compile because MyPluginName isn't const
[Plugin(MyConstPluginName)] // OK
[Plugin("My Cool Plugin")] // OK
In order to access properties on the static class, it must specify the name of the class before the properties name as proposed on this change:
[Plugin(Variables.MyPluginName)] // Won't compile because MyPluginName isn't const
[Plugin(Variables.MyConstPluginName)] // OK
[Plugin("My Cool Plugin")] // OK
Second on access on attribute properties:
In order to access plugin.Name on this code:
var type = typeof(MyPlugin); // Returns a Type object representing our MyPlugin class
var attributes = System.Attribute.GetCustomAttributes(type); // Returns an Attribute[]
foreach (var a in attributes)
{
if (a is PluginAttribute plugin)
Console.WriteLine($"Plugin Name: {plugin.Name}");
}
You need to cast if first to the right class, then access the properties, as the above code will generate a compile-time error:
var type = typeof(PluginAttribute); // Returns a Type object representing our PluginAttribute class
var attributes = System.Attribute.GetCustomAttributes(type); // Returns an Attribute[]
foreach (var a in attributes)
{
if (a is PluginAttribute)
{
PluginAttribute plug = (PluginAttribute)a; //Cast it first to PluginAttribute class before you can access all accessible properties
Console.WriteLine($"Plugin Name: {plug.Name}");
}
}
72 lines
2.7 KiB
Markdown
72 lines
2.7 KiB
Markdown
---
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title: Attributes
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---
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# Attributes
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Attributes allow the programmer to add metadata to assemblies, types, and members. When applied to a type or member, an attribute will take the form of `[Attribute]` or `[Attribute(args)]`. When applied to an assembly, an attribute will take the form of `[assembly:Attribute]` or `[assembly:Attribute(args)]`.
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An attribute is any class that inherits from the parent `Attribute` type. Conventionally, the name of an attribute in its definition will be `SomethingAttribute`, rather than just `Something`. When used, `[Something]` will still compile:
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```csharp
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public class PluginAttribute : Attribute
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{
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// Attribute members
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}
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[Plugin]
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public class MyPlugin
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{
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// Plugin members
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}
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```
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As mentioned above, attributes can have constructors like any other class. However, all arguments in an attribute constructor must be compile-time constants.
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```csharp
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public static class Variables // For the purposes of demonstration
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{
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public static string MyPluginName = "Cool Plugin";
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public const string MyConstPluginName = "Amazing Plugin";
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}
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public class PluginAttribute : Attribute
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{
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public string Name { get; private set; }
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public PluginAttribute(string name)
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{
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Name = name;
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}
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}
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[Plugin(Variables.MyPluginName)] // Won't compile because MyPluginName isn't const
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[Plugin(Variables.MyConstPluginName)] // OK
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[Plugin("My Cool Plugin")] // OK
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public class MyPlugin
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{
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// Plugin members
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}
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```
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## Accessing a Type's Attributes
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The `System.Attribute.GetCustomAttributes(Type)` method returns an array of all the attributes applied to a type. The programmer can then loop through this array to find the desired attribute using the `is` keyword.
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```csharp
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public void PrintPluginName()
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{
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var type = typeof(PluginAttribute); // Returns a Type object representing our PluginAttribute class
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var attributes = System.Attribute.GetCustomAttributes(type); // Returns an Attribute[]
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foreach (var a in attributes)
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{
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if (a is PluginAttribute)
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{
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PluginAttribute plug = (PluginAttribute) a; //Cast it first to PluginAttribute class before you can access all accessible properties
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Console.WriteLine($"Plugin Name: {plug.Name}");
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}
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}
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}
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```
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#### Additional Information
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* [Attributes - Microsoft Programming Guide](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/attributes/)
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* [Accessing Attributes Using Reflection - Microsoft Programming Guide](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/attributes/accessing-attributes-by-using-reflection)
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* [Creating Custom Attributes - Microsoft Programming Guide](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/attributes/creating-custom-attributes)
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