What are the ways to declare and define an attribute in C#?

In C#, attributes are special classes that allow you to add metadata to code elements (classes, methods, properties, etc.). To declare and define your own attribute, you need to:

1. Create a class that inherits from System.Attribute.
2. By convention, the attribute class name ends with Attribute, but this part can be omitted when applying it.
3. In the class, you can define constructors and properties to pass parameters to the attribute.

Example of declaring a simple attribute:

using System;

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class MyCustomAttribute : Attribute
{
    public string Description { get; }

    public MyCustomAttribute(string description)
    {
        Description = description;
    }
}

// Using the attribute:
[MyCustom("This is my attribute")]
public class SampleClass
{
    [MyCustom("Method with attribute")]
    public void SampleMethod() { }
}

Thus, declaring an attribute involves creating a subclass of Attribute, and defining it involves adding the necessary constructors and properties to pass data.