Question

What is the right way to do this? This way is not working:

if (((String)enterAmountButton.Content) == "")
 MessageBox.Show("Please enter the total bill amount.");
else if (((String)enterTipButton.Content) == "")
 MessageBox.Show("Please enter the tip % amount.");   

this way is not working neither:

if (enterAmountButton.Content == "")   
 MessageBox.Show("Please enter the total bill amount.");   
else if (enterTipButton.Content == "")   
 MessageBox.Show("Please enter the tip % amount.");   

Answer 1

var amountButtonString = enterAmountButton.Content as string;
        var enterTipButtonString = enterTipButton.Content as string;
        if (String.IsNullOrEmpty(amountButtonString))
            MessageBox.Show("Please enter the total bill amount.");
        else if (String.IsNullOrEmpty(enterTipButtonString))
            MessageBox.Show("Please enter the tip % amount.");  

would work. However, how are you expecting to get the strings? you most likely want the value in a TextBox that is next to the button, correct?

in that case:

if (String.IsNullOrEmpty(amountTextBox.Text))
                MessageBox.Show("Please enter the total bill amount.");
            else if (String.IsNullOrEmpty(tipTextBox.Text))
                MessageBox.Show("Please enter the tip % amount.");  

where amountTextBox & tipTextBox are the x:Name of your TextBoxes.

One last thing:

There’s probably a better way to handle this. For example, if you handle the KeyUp & TextChanged events on the TextBoxes and only Enable the button when text exists (even better, valid text ;))

You could use a converter too:

public class StringToEnabledConverter : IValueConverter
    {
        #region IValueConverter Members

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            var val = value as string;
            if (val == null)
                throw new ArgumentException("value must be a string.");

            return !string.IsNullOrEmpty(val);
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }

        #endregion
    }

Add a reference to your converter <Converters:StringToEnabledConverter x:Key="StringToEnabledConverter" />

and on your button,

<TextBox x:Name="amountTextBox />
<Button x:Name="enterAmountButton" Enabled="{Binding ElementName=amountTextBox, Path=Text, Converter={StaticResource StringToEnabledConverter}}" />