public class Foo : IDisposable
{
    public void Dispose()
    {
        Console.WriteLine("Dispose called!");
    }
}

using(var foo = new Foo())
{
    // ... do stuff
}

var foo = new Foo()
try
{
    // ... do stuff
}
finally
{
    if (foo != null)
        ((IDisposable)foo).Dispose();
}

Dispose called!

public class Foo : IDisposable
{
    public void Dispose()
    {
        Console.WriteLine("Dispose called!");
    }

    ~Foo()
    {
        Dispose();
    }
}

public class Foo : IDisposable
{
    public void Dispose()
    {
        RealDispose();
        GC.SuppressFinalize(this);
    }

    protected void RealDispose()
    {
        Console.WriteLine("Dispose called!");
        // тут будет сама логика освобождения ресурса
    }

    ~Foo()
    {
        RealDispose();
    }
}

public class Foo : IDisposable
{
    SqlConnection conn;

    public Foo()
    {
        conn = new SqlConnection(....);
    }

    public void Dispose()
    {
        conn?.Dispose();
    }
}