Recently, I built a simple validation framework which can work with WPF. I found I can use class attribute and reflection to achieve this purpose.
Firstly, I define a Required Attribute to see whether the property requires a value

Code

Then I add IsValid method in the class, it will use the reflect to get all properties in this class. Then it will check whether the property has "required=true", if that is the case, it will check whether the property has value, if not, it will use an error.

public class PersonModel
	{
		[Required(true)]
		public string Name {get;set;}
		public IList IsValid()
		{
			IList ret = new List();
			foreach (var prop in this.GetType().GetProperties())
			{
				foreach(Attribute attr1 in  prop.GetCustomAttributes(typeof(RequiredAttribute),true)){
					RequiredAttribute attr = (RequiredAttribute) attr1;
					if (attr != null) {
						if (attr.Required)
						{
							if (string.IsNullOrEmpty(Convert.ToString(prop.GetValue(this, null)))) {
								ret.Add(string.Format("{0} is required",prop.Name));
							}
							
						}
					}
				}

			}
			return ret;
		}
	}

is the best practice. I cannot think another way can do it easier.
Please click here to download the example code.

I got an app to call api in my server for loading the data. This make sense to use asynchronous methods for loading the data. I don't want the whole UI is locked for waiting the data. But I found
model.GetData().ContinueWith((items) =>
{
list.ItemSource= items;
});

It won't works. I found the reason is all Asynchronous methods are executed by a different thread. You have to call UI main thread to bind the data to UI.
model.GetData().ContinueWith((items) =>
{
evice.BeginInvokeOnMainThread(() =>
{
list.ItemSource= items;
});
});;

After I installed Facebook SDK for Andorid. I found I cannot deploy my app in the debug anymore. I clicked the play button in Visual Studio, no compilation errors at all. Visual Studio is only freezed in "Deploying in [Phone Name]" for a little white. Then it stoped and gave out this error message,'Exception in thread "main"'. After I did a number of random search in Google, I found some people in the Xamarin Forum mentioned they need to increase the heap size to 1G after installing Facebook SDK. Finally, I increased the heap size to 1G in (Project Properties -> Android Options->Advanced-> Java Max Heap Size). Then it works!

After I switched my Xamarin Forms app from Normal Form Activity to FormsAppCompatActivity. This can provide more function in Material Design, such as TextInputLayout. But I got this error:System.MissingMethodException: Method 'Android.Support.V4.Widget.DrawerLayout.AddDrawerListener' not found. I found the problem is from the libraries. After I updated the latest Xamarin Forms Library in Android Project. That works!

I got an error when I built my application. The error said 'No xmlns declaration for prefix "local". Because I used the xaml, I found the compiler did not check the error in xaml file. As a result, it got the runtime error like. This is very easy to solve. Please search all xaml files you used local has this xmlns:local="clr-namespace:[your namespace]"