c# mssql injection error

// assuming that the below code is in a on button click method, what would be the best way of making sure a user cannot inject code into the username textbox? (it may be in the "SqlDataAdapter" line)

SqlConnection sqlConnection = new SqlConnection("Data Source=mssqlinstllation\\databasename;Initial Catalog=TestLogin;Integrated Security=True");
SqlDataAdapter sqlDataAdapter = new SqlDataAdapter("Select Count(*) From LoginTable where Username= '" + textBoxUsername.Text + "' and Password = '" + textBoxPassword.Text + "'", sqlConnection);
DataTable dataTable = new DataTable();
sqlDataAdapter.Fill(dataTable);

if (dataTable.Rows[0][0].ToString() == "1")
{
    MainForm mainForm = new MainForm();
    mainForm.Show();
    this.Hide();
}
else // if it is not in database
{
    MessageBox.Show("The Username and/or Password are incorrect!", "Invalid login infomation!", MessageBoxButtons.OK, MessageBoxIcon.Error);
}