CustomMsgBox.cs

using System;
using System.ComponentModel;
using System.Drawing;
using System.Windows.Forms;

public enum WinVersionStyle {
    WinXP,
    Win7,
    Win10
}

public partial class CustomMsgBoxForm : Form {
    public CustomMsgBoxForm() {
        Application.EnableVisualStyles();
        InitializeComponent();
    }

    public string Prompt {
        get => txtMain.Text;
        set => txtMain.Text = value;
    }

    public string Title {
        get => Text;
        set => Text = value;
    }

    public TextBox MainText => txtMain;

    public MessageBoxIcon FormLevel { get; set; }
    private string getFormLevelString() {
        return FormLevel switch {
            MessageBoxIcon.Error => "Error",
            MessageBoxIcon.Question => "Question",
            MessageBoxIcon.Exclamation => "Exclamation",
            MessageBoxIcon.Information => "Information",
            _ => ""
        };
    }

    public string Button1Text = null;
    public string Button2Text = null;
    public string Button3Text = null;
    public WinVersionStyle WinVersion = WinVersionStyle.Win10;
    public string DialogResultString = null;
    public byte DialogResultButtonPressed = 0;

    public MessageBoxButtons Buttons {
        set {
            switch (value) {
                case MessageBoxButtons.OK:
                    Button1Text = "OK";
                    break;
                case MessageBoxButtons.OKCancel:
                    Button1Text = "OK";
                    Button3Text = "Cancel";
                    break;
                case MessageBoxButtons.AbortRetryIgnore:
                    Button1Text = "Abort";
                    Button2Text = "Retry";
                    Button3Text = "Ignore";
                    break;
                case MessageBoxButtons.YesNoCancel:
                    Button1Text = "Yes";
                    Button2Text = "No";
                    Button3Text = "Cancel";
                    break;
                case MessageBoxButtons.YesNo:
                    Button1Text = "Yes";
                    Button3Text = "No";
                    break;
                case MessageBoxButtons.RetryCancel:
                    Button1Text = "Retry";
                    Button3Text = "Cancel";
                    break;
            }
        }
    }

    private void CustomMsgBoxForm_Load(object _, EventArgs __) {
        if (string.IsNullOrEmpty(Text)) {
            try {
                Text = Owner.Text;
            } catch {
                Text = "CustomMsgBox";
            }
        }

        var resources = new ComponentResourceManager(typeof(CustomMsgBoxForm));
        Icon = (Icon)resources.GetObject(WinVersion.ToString() + "_" + getFormLevelString());
        if (FormLevel != MessageBoxIcon.None)
            pbxMain.Image = Icon.ToBitmap();
        try {
            Icon = Owner.Icon;
        } catch {
            // Icon = pbxMain.Image;
            // doesn't work, and it's already set above anyway
            if (FormLevel == MessageBoxIcon.None)
                ShowIcon = false;
        }

        switch (FormLevel) {
            case MessageBoxIcon.Error:
                System.Media.SystemSounds.Beep.Play();
                break;
            case MessageBoxIcon.Exclamation:
                System.Media.SystemSounds.Exclamation.Play();
                break;
            case MessageBoxIcon.Information:
                System.Media.SystemSounds.Asterisk.Play();
                break;
            case MessageBoxIcon.Question:
                System.Media.SystemSounds.Question.Play();
                break;
        }

        // as TextBox doesn't have an AutoSize property like Label does, we have to do it manually
        using (Graphics g = txtMain.CreateGraphics()) {
            SizeF sizeF = g.MeasureString(txtMain.Text, txtMain.Font, new SizeF(txtMain.MaximumSize.Width, float.MaxValue));
            txtMain.Height = (int)Math.Ceiling(sizeF.Height / txtMain.Font.Height) * txtMain.Font.Height; // restrain to line height
        }

        if (txtMain.Height > 13)
            Height = 162 + (txtMain.Height - 13);

        bool showB1 = !string.IsNullOrWhiteSpace(Button1Text);
        bool showB2 = !string.IsNullOrWhiteSpace(Button2Text);
        bool showB3 = !string.IsNullOrWhiteSpace(Button3Text);

        btnAccept.Visible = showB1;
        btnAccept.Text = Button1Text;
        btnAnswerMid.Visible = showB2;
        btnAnswerMid.Text = Button2Text;
        btnCancel.Visible = showB3;
        btnCancel.Text = Button3Text;

        int getButtonRightX(Button btn) => btn.Location.X + btn.Width;
        void setButtonX(Button btn, int x) => btn.Location = new Point(x, btn.Location.Y);
        const int maxTotalWidth = 242;
        const int buttonSpacing = 8;

        int currentTotalWidth = (showB1 ? btnAccept.Width : 0) + (showB2 ? btnAnswerMid.Width : 0) + (showB3 ? btnCancel.Width : 0);
        if (showB1 && showB2 && showB3)
            currentTotalWidth += buttonSpacing * 2;
        else if ((showB1 && showB2) || (showB1 && showB3) || (showB2 && showB3))
            currentTotalWidth += buttonSpacing;


        if (currentTotalWidth <= maxTotalWidth) { // if enough space, align to left point
            const int button1LeftStartX = 152;

            if (showB1)
                setButtonX(btnAccept, button1LeftStartX);

            if (showB1 && showB2)
                setButtonX(btnAnswerMid, getButtonRightX(btnAccept) + buttonSpacing);
            else if (showB2)
                setButtonX(btnAnswerMid, button1LeftStartX);

            if (showB2 && showB3)
                setButtonX(btnCancel, getButtonRightX(btnAnswerMid) + buttonSpacing);
            else if (showB1 && showB3)
                setButtonX(btnCancel, getButtonRightX(btnAccept) + buttonSpacing);
            else if (showB3)
                setButtonX(btnCancel, button1LeftStartX);
        } else { // if not enough space, align right
            const int button3rightX = 393;

            if (showB3)
                setButtonX(btnCancel, button3rightX - btnCancel.Width);

            if (showB3 && showB2)
                setButtonX(btnAnswerMid, btnCancel.Location.X - btnAnswerMid.Width - buttonSpacing);
            else if (showB2)
                setButtonX(btnAnswerMid, button3rightX - btnAnswerMid.Width);

            if (showB2 && showB1)
                setButtonX(btnAccept, btnAnswerMid.Location.X - btnAccept.Width - buttonSpacing);
            else if (showB3 && showB1)
                setButtonX(btnAccept, btnCancel.Location.X - btnAccept.Width - buttonSpacing);
            else if (showB1)
                setButtonX(btnAccept, button3rightX - btnAccept.Width);
        }

        if (Owner != null)
            CenterToParent();
        else
            CenterToScreen();
        btnAccept.Select();
    }

    private DialogResult GetDialogResult(string buttonText) =>
        Enum.TryParse(buttonText, true, out DialogResult result) ? result : DialogResult.None;

    private void btnAccept_Click(object _, EventArgs __) {
        DialogResult = GetDialogResult(Button1Text);
        DialogResultString = Button1Text; // for use with custom buttons
        DialogResultButtonPressed = 1;
        if (GetDialogResult(Button1Text) == DialogResult.None)
            Close();
    }

    private void btnAnswerMid_Click(object _, EventArgs __) {
        DialogResult = GetDialogResult(Button2Text);
        DialogResultString = Button2Text;
        DialogResultButtonPressed = 2;
        if (GetDialogResult(Button2Text) == DialogResult.None)
            Close();
    }

    private void btnCancel_Click(object _, EventArgs __) {
        DialogResult = GetDialogResult(Button3Text);
        DialogResultString = Button3Text;
        DialogResultButtonPressed = 3;
        if (GetDialogResult(Button3Text) == DialogResult.None)
            Close();
    }
}

public partial class WalkmanLib {
    /// <summary>Shows a custom messagebox</summary>
    /// <param name="text">The text to display in the message box.</param>
    /// <param name="caption">The text to display in the title bar of the message box.</param>
    /// <param name="buttons">One of the <see cref="MessageBoxButtons"/> values that specifies which buttons to display in the message box.</param>
    /// <param name="style">One of the following: <see cref="MessageBoxIcon.Error"/>, <see cref="MessageBoxIcon.Question"/>, <see cref="MessageBoxIcon.Exclamation"/> or <see cref="MessageBoxIcon.Information"/>.</param>
    /// <param name="winVersion">Windows version to use style icons from. Default: <see cref="WinVersionStyle.Win10"/></param>
    /// <param name="ownerForm">Used to set the Window's Icon. Set to <see langword="this"/> to copy the current form's icon</param>
    /// <returns>The button the user selected, or <see cref="DialogResult.Cancel"/> if Window X button selected.</returns>
    public static DialogResult CustomMsgBox(string text, string caption = null, MessageBoxButtons buttons = 0, MessageBoxIcon style = 0,
                                            WinVersionStyle winVersion = WinVersionStyle.Win10, Form ownerForm = null) {
        var formToShow = new CustomMsgBoxForm() {
            Prompt = text,
            Title = caption,
            Buttons = buttons,
            FormLevel = style,
            WinVersion = winVersion,
            Owner = ownerForm,
            ShowInTaskbar = false
        };
        return formToShow.ShowDialog();
    }

    /// <summary>Shows a custom messagebox with custom buttons</summary>
    /// <param name="text">The text to display in the message box.</param>
    /// <param name="caption">The text to display in the title bar of the message box.</param>
    /// <param name="customButton1">Text to show on the first button</param>
    /// <param name="customButton2">Text to show on the second button. If left out or set to <see langword="null"/>, this button will be hidden.</param>
    /// <param name="customButton3">Text to show on the third button. If left out or set to <see langword="null"/>, this button will be hidden.</param>
    /// <param name="style">One of the following: <see cref="MessageBoxIcon.Error"/>, <see cref="MessageBoxIcon.Question"/>, <see cref="MessageBoxIcon.Exclamation"/> or <see cref="MessageBoxIcon.Information"/>.</param>
    /// <param name="winVersion">Windows version to use style icons from. Default: <see cref="WinVersionStyle.Win10"/></param>
    /// <param name="ownerForm">Used to set the Window's Icon. Set to <see langword="this"/> to copy the current form's icon</param>
    /// <returns>Text of the button the user selected, or <see langword="null"/> if Window X button selected.</returns>
    public static string CustomMsgBox(string text, string caption, string customButton1, string customButton2 = null, string customButton3 = null,
                                      MessageBoxIcon style = 0, WinVersionStyle winVersion = WinVersionStyle.Win10, Form ownerForm = null) {
        var formToShow = new CustomMsgBoxForm() {
            Prompt = text,
            Title = caption,
            Button1Text = customButton1,
            Button2Text = customButton2,
            Button3Text = customButton3,
            FormLevel = style,
            WinVersion = winVersion,
            Owner = ownerForm,
            ShowInTaskbar = false
        };

        formToShow.ShowDialog();
        return formToShow.DialogResultString;
    }


    /// <returns>Which button the user selected: 1-3. 0 = Window X button selected.</returns>
    /// <inheritdoc cref="CustomMsgBox(string, string, string, string, string, MessageBoxIcon, WinVersionStyle, Form)"/>
    public static byte CustomMsgBoxBTN(string text, string caption, string customButton1, string customButton2 = null, string customButton3 = null,
                                       MessageBoxIcon style = 0, WinVersionStyle winVersion = WinVersionStyle.Win10, Form ownerForm = null) {
        var formToShow = new CustomMsgBoxForm() {
            Prompt = text,
            Title = caption,
            Button1Text = customButton1,
            Button2Text = customButton2,
            Button3Text = customButton3,
            FormLevel = style,
            WinVersion = winVersion,
            Owner = ownerForm,
            ShowInTaskbar = false
        };

        formToShow.ShowDialog();
        return formToShow.DialogResultButtonPressed;
    }
}

// visual styles is enabled in a Windows Forms Application with the following:
//static void Main() {
//    Application.EnableVisualStyles();
//    ...
//}