C#C
C#•3y ago
doolhofx

❔ Need help with Event Handlers to update a field.

I am making a unit conversion program in C# wpf as my first C# application. I have created a method that calculates the conversion from for example mm to m. It has 3 inputs, Input Unit - Output Unit - Input Value. Calculates the Output Value.

I want to update the Output Value on 3 different events, InputWaarde_TextChanged or InputEenheid_SelectionChanged or OutputEenheid_SelectionChanged.
But when I have the method in those event handlers I get the following error when I start the program and select a unit, both input or output or when I fill in a value. 
System.NullReferenceException
System.Windows.Controls.Primitives.Selector.SelectedItem.get returned null.


Anyone has an idea on how to solve it? And is anyone willing to look at the rest of the code to give feedback? I think I can use some haha!

        private void InputWaarde_TextChanged(object sender, TextChangedEventArgs e)
        {
            OutputWaarde.Text = Formules.Lengte(InputEenheid.SelectedItem.ToString(), OutputEenheid.SelectedItem.ToString(), InputWaarde.Text);

        }
        
        private void InputEenheid_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            OutputWaarde.Text = Formules.Lengte(InputEenheid.SelectedItem.ToString(), OutputEenheid.SelectedItem.ToString(), InputWaarde.Text);

        }

        private void OutputEenheid_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            OutputWaarde.Text = Formules.Lengte(InputEenheid.SelectedItem.ToString(), OutputEenheid.SelectedItem.ToString(), InputWaarde.Text);

        }
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