public void button2_Click(object sender, System.EventArgs e)
{
    string text = textBox1.Text;
    Mainform = this;

    this.Hide();

    GetSchedule myScheduleFinder = new GetSchedule();
    string result = myScheduleFinder.GetDataFromNumber(text);// says there is no definition
    if (!string.IsNullOrEmpty(result))
    {
        MessageBox.Show(result);
    }
    else
    {
        MessageBox.Show("Enter A Valid ID Number!");
    }
}

says it does not contain definition for it but on my GetSchedule .cs file i have it defined

public string GetDataFromNumber(string ID)//defined here
{

    foreach (IDnumber IDCandidateMatch in IDnumbers)
    {

        if (IDCandidateMatch.ID == ID)
        {
            StringBuilder myData = new StringBuilder();
            myData.AppendLine(IDCandidateMatch.Name);
            myData.AppendLine(": ");
            myData.AppendLine(IDCandidateMatch.ID);
            myData.AppendLine(IDCandidateMatch.year);
            myData.AppendLine(IDCandidateMatch.class1);
            myData.AppendLine(IDCandidateMatch.class2);
            myData.AppendLine(IDCandidateMatch.class3);
            myData.AppendLine(IDCandidateMatch.class4);
            //return myData;
            return myData.ToString();
        }
    }
    return "";
}