TextBoxWithMessage.cs

class TextBoxWithMessage 
{
	private TextBox textbox;
	private String message;

	public TextBoxWithError(TextBox textbox, String message) {
		this.textbox = textbox;
		this.message = message;
	}

	public TextBox getTextBox() {
		return textbox;
	}

	public String getMessage() {
		return message;
	}
}

TextInputValidator.cs

/**
	TextInputValidator v = new TextInputValidator();

	Set the rules for the type of input for the textbox that is handled by the keypressed event:
	v.forInt(myIntTextBox,"Textbox 'myIntTextBox' should not be empty");
	v.forString(myStringTextBox,"Textbox 'myStringTextBox' should not be empty");
	v.forStringInt(myStringIntTextBox,"Textbox 'myStringIntTextBox' should not be empty");

	When submitting the values use:
	v.onSubmitPassed();
*/
public class TextInputValidator
{
	private List<TextBoxWithMessage> textboxList = new List<TextBoxWithMessage>();
	public TextInputValidator() {}

	public void forInt(TextBoxWithMessage textbox) {
		if(textbox.getTextBox() != null) {
			textboxList.Add(textbox);
			textbox.getTextBox().KeyPress += new KeyPressEventHandler(forIntKeyPressed);
		}
	}

	public void forString(TextBoxWithMessage textbox) {
		if(textbox.getTextBox() != null) {
			textboxList.Add(textbox);
			textbox.getTextBox().KeyPress += new KeyPressEventHandler(forStringKeyPressed);
		}
	}

	public void forStringInt(TextBoxWithMessage textbox) {
		if(textbox.getTextBox() != null) {
			textboxList.Add(textbox);
			textbox.getTextBox().KeyPress += new KeyPressEventHandler(forStringIntKeyPressed);
		}
	}

	public void forIntKeyPressed(Object o, KeyPressEventArgs e) {
		if (!char.IsControl(e.KeyChar) && !char.IsDigit(e.KeyChar)) { 
			e.Handled = true; 
		}
	}

	public void forStringKeyPressed(Object o, KeyPressEventArgs e) {
		if (!char.IsControl(e.KeyChar) && !char.IsLetter(e.KeyChar)) { 
			e.Handled = true; 
		}
	}

	public void forStringIntKeyPressed(Object o, KeyPressEventArgs e) {
		if (!char.IsControl(e.KeyChar) && !char.IsLetter(e.KeyChar) && !char.IsDigit(e.KeyChar)) { 
			e.Handled = true; 
		}
	}

	public boolean onSubmitPassed() {
		foreach(TextBoxWithMessage textbox in textboxList) {
			if(string.IsNullOrEmpty(textbox.getTextBox().Text)) {
				 MessageBox.Show(textbox.getMessage(), "Error!", MessageBoxButtons.OK, MessageBoxIcon.Error);
				 return false;
			}
		}
		return true;
	}
}