C#

CREATE TABLE table1
(
Reci nvarchar(13)
);

INSERT INTO table1 VALUES ('заступниство');
INSERT INTO table1 VALUES ('затровавање');
INSERT INTO table1 VALUES ('заустављати');
INSERT INTO table1 VALUES ('заварљивост');
INSERT INTO table1 VALUES ('земљорадник');
INSERT INTO table1 VALUES ('земљорадња');
INSERT INTO table1 VALUES ('згњецавање');
INSERT INTO table1 VALUES ('згусњавање');
INSERT INTO table1 VALUES ('злонамерност');
INSERT INTO table1 VALUES ('злостављати');
INSERT INTO table1 VALUES ('зверокрадица');
.........

string cs= @"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\Pavle\Documents\Visual Studio 2015\Projects\Test slagalica\Test slagalica\Slagalica-DB-Cirilica.mdf;Integrated Security=True";
        string queryString = "SELECT * FROM table1 WHERE LEN(Reci) >=10 AND LEN(Reci) <=12 ORDER BY NEWID()";
using (SqlConnection connection = new SqlConnection(cs))
            {
                SqlCommand mycommand = new SqlCommand(queryString, connection);
                try
                {
                    connection.Open();
                    string word = (string)mycommand.ExecuteScalar();
                    label14.Text = word;
                    promesana_rec = new string(word.ToUpper().OrderBy(r => random.Next()).ToArray());
                    textBox2.Text = promesana_rec.ToUpper();
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }
            }

INSERT INTO table1 (Reci) VALUES (N'заступниство');

CREATE TABLE table1
(
ID [INT] IDENTITY(1,1) NOT NULL,
Reci nvarchar(13) NOT NULL,
WordLen INT NOT NULL
);

SELECT TOP 1 Reci FROM Table1 ...

using (SqlConnection connection = new SqlConnection(cs))
                {
                    SqlCommand cmd = new SqlCommand("SELECT * FROM table1 WHERE Reci='" + textBox1.Text + "'", connection);
                    try
                    {
                        connection.Open();
                        SqlDataAdapter da = new SqlDataAdapter(cmd);
                        DataSet ds = new DataSet();
                        da.Fill(ds);
                        int j = ds.Tables[0].Rows.Count;
                        if (j == 0)
                        {
                            MessageBox.Show("word" + textBox1.Text + "already exists!");
                            ds.Clear();
                        }
                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show(ex.Message);
                    }
                }

private void uxRunCount_PreviewKeyDown( object sender, System.Windows.Input.KeyEventArgs e ) {

   bool ok = IsNumeric( e.Key );

   if ( !ok ) {
      e.Handled = true;    // Reject change.
   }
}

private bool IsNumeric( Key key ) {

   if ( key == Key.Back || key == Key.Delete ) { return true; }
   if ( key == Key.Tab || key == Key.Return ) { return true; }

   if ( key >= Key.D0 && key <= Key.D9 ) { return true; }
   if ( key >= Key.NumPad0 && key <= Key.NumPad9 ) { return true; }

   return false;
}

private void myTextBox_KeyPress(object sender, KeyPressEventArgs e)

char keyPressedByUser = e.KeyChar;

var allowedKeys = promesana_rec.Select(ch => (Keys)Convert.ToInt32(char.ToUpper(ch)));

e.SuppressKeyPress = !allowedKeys.Contains(e.KeyCode) && new[] { Keys.None, Keys.Shift }.Contains(e.Modifiers);

private string theWord = "extemporized".ToUpper();
...
private void myTextBox_KeyPress(object sender, KeyPressEventArgs e)
    {
    char keyPressedByUser = char.ToUpper(e.KeyChar);
    if (!theWord.Contains(keyPressedByUser))
        {
        e.Handled = true;
        return;
        }
    ...
    }

private Dictionary<char, int> letters;

letters = theWord.GroupBy(c => c).ToDictionary(g => g.Key, g => g.Count());

private void myTextBox_KeyPress(object sender, KeyPressEventArgs e)
    {
    if (char.IsLetter(e.KeyChar))
        {
        char keyPressedByUser = char.ToUpper(e.KeyChar);
        if (!theWord.Contains(keyPressedByUser))
            {
            e.Handled = true;
            return;
            }
        int inputCount = myTextBox.Text.ToUpper().Where(c => c == keyPressedByUser).Count() + 1;
        if (inputCount > letters[keyPressedByUser])
            {
            e.Handled = true;
            return;
            }
        e.Handled = false;
        }
    }