Alerts

Display an Alert

For Alerts since iOS 8, you would use a UIAlertController but for versions before, you would have used a UIAlertView, which is now deprecated.

var alert = UIAlertController.Create(title, message, UIAlertControllerStyle.Alert);
alert.AddAction(UIAlertAction.Create(otherTitle, UIAlertActionStyle.Destructive, (action) => {
    // otherTitle();
}));
alert.AddAction(UIAlertAction.Create(cancelTitle, UIAlertActionStyle.Cancel, null));
this.PresentViewController(alert, true, null);

var alert = new UIAlertView (title, message, null, cancelTitle, otherTitle);
alert.Clicked += (object sender, UIButtonEventArgs e) => {
    if(e.ButtonIndex == 1)
       // otherTitle();
};
alert.Show ();

Display a login alert

The following code is for iOS 8 and lower for creating a login alert.

// Create the UIAlertView
var loginAlertView = new UIAlertView(title, message, null, cancelTitle, okTitle);

// Setting the AlertViewStyle to UIAlertViewStyle.LoginAndPasswordInput
loginAlertView.AlertViewStyle = UIAlertViewStyle.LoginAndPasswordInput;

// Getting the fields Username and Password
var usernameTextField = loginAlertView.GetTextField(0);
var passwordTextField = loginAlertView.GetTextField(1);

// Setting a placeholder
usernameTextField.Placeholder = "user@stackoverflow.com";
passwordTextField.Placeholder = "Password";

// Adding the button click handler.
loginAlertView.Clicked += (alertViewSender, buttonArguments) =>
{
    
    // Check if cancel button is pressed
    if (buttonArguments.ButtonIndex == loginAlertView.CancelButtonIndex)
    {
        // code
    }
    
    // In our case loginAlertView.FirstOtherButtonIndex is equal to the OK button
    if (buttonArguments.ButtonIndex == loginAlertView.FirstOtherButtonIndex)
    {
        // code
    }
};

// Show the login alert dialog
loginAlertView.Show();

Display an Action Sheet

The UIAlertController available since iOS8 allows you to use the same alert object for either Action sheets or more classic alerts. The only difference is the UIAlertControllerStyle passed as a parameter when creating.

This line changes from an AlertView to an ActionSheet, compared to some other examples available here :

var alert = UIAlertController.Create(title, message, UIAlertControllerStyle.ActionSheet);

The way you add actions to the controller is still the same :

alert.AddAction(UIAlertAction.Create(otherTitle, UIAlertActionStyle.Destructive, (action) => {
    // ExecuteSomeAction();
}));
alert.AddAction(UIAlertAction.Create(cancelTitle, UIAlertActionStyle.Cancel, null));

//Add additional actions if necessary

Note that if you have a parameterless void method, you can use it as the last parameter of the .AddAction().

For example, let's assume I want the code of private void DoStuff(){...} to be executed when I press "OK" :

UIAlertAction action = UIAlertAction.Create("OK", UIAlertActionStyle.Cancel, DoStuff);
alert.AddAction(action);

Notice I'm not using the () after DoStuff in the creation of the action.

The way you present the controller is done the same way as any other controller :

this.PresentViewController(alert, true, null);

Display Modal Alert Dialog

It was common practice to use NSRunLoop to show modal UIAlertView to block code execution until user input is processed in iOS; until Apple released the iOS7, it broke few existing apps. Fortunately, there is a better way of implementing it with C#’s async/await.

Here’s the new code taking advantage of async/await pattern to show modal UIAlertView:

Task ShowModalAletViewAsync (string title, string message, params string[] buttons)
{
    var alertView = new UIAlertView (title, message, null, null, buttons);
    alertView.Show ();
    var tsc = new TaskCompletionSource ();

    alertView.Clicked += (sender, buttonArgs) => {
        Console.WriteLine ("User clicked on {0}", buttonArgs.ButtonIndex);
        tsc.TrySetResult(buttonArgs.ButtonIndex);
    };
    return tsc.Task;
}

//Usage
async Task PromptUser() {
    var result = await ShowModalAletViewAsync 
               ("Alert", "Do you want to continue?", "Yes", "No"); //process the result
}