public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        String symbols = textBoxFrom.Text;

        String result = Form1.maxLenghWordContainsSymbols(symbols);

        textBoxOutWord.Text = result;
    }

    private static bool maxLenghWordContainsSymbols(String symbols)
    {
        String[] words = symbols.ToLower().Split(' ', StringSplitOptions.RemoveEmptyEntries);

        int maxLengthWordIndex = 0, maxLengthWord = words[0].Length;
        for (int i = 1; i < words.Length; i++)
        {
            if (words[i].Length > maxLengthWord) {
                maxLengthWordIndex = i;
                maxLengthWord = words.Length;
            }
        }

        if (words[maxLengthWordIndex].Length % 2 == 1) {
            int middleIndex = (words[maxLengthWordIndex].Length - 1) / 2;
            return words[maxLengthWordIndex].Remove(middleIndex, 1);
        }

        return "Самое длинное сообщение чётной длины";
    }

}