---------
IndexOf(value, startIndex, count)
從指定索引開始且包含指定個數項目的範圍內，搜尋指定的 Object 第一次出現的位置，並傳回其索引值 (從零開始起算)。 
如果 count 大於 0， ArrayList 是從 startIndex 開始向前搜尋，在 startIndex 加 count 減 1 結束。 
這個方法會執行線性搜尋；因此，這個方法是 O( n) 運算，其中 n 是 count。 
這個方法會呼叫 Object.Equals 來判斷相等性。 
從 .NET Framework 2.0 開始，這個方法就會在 item 上使用集合物件的 Equals 和 CompareTo 方法，以判斷項目是否存在。 在舊版的 .NET Framework 中，會在此集合之物件上使用 item 參數的 Equals 和 CompareTo 方法來做出這項決定。


using System;
using System.Collections;
public class SamplesArrayList
{

    public static void Main()  
    {

        // Creates and initializes a new ArrayList with three elements of the same value.
        ArrayList myAL = new ArrayList();
        myAL.Add( "the" );
        myAL.Add( "quick" );
        myAL.Add( "brown" );
        myAL.Add( "fox" );
        myAL.Add( "jumps" );
        myAL.Add( "over" );
        myAL.Add( "the" );
        myAL.Add( "lazy" );
        myAL.Add( "dog" );
        myAL.Add( "in" );
        myAL.Add( "the" );
        myAL.Add( "barn" );

        // Displays the values of the ArrayList.
        Console.WriteLine( "The ArrayList contains the following values:" );
        PrintIndexAndValues( myAL );

        // Search for the first occurrence of the duplicated value.
        String myString = "the";
        int myIndex = myAL.IndexOf( myString );
        Console.WriteLine( "The first occurrence of \"{0}\" is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in the last section of the ArrayList.
        myIndex = myAL.IndexOf( myString, 4 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 4 and the end is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in a section of the ArrayList.
        myIndex = myAL.IndexOf( myString, 6, 6 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 6 and index 11 is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in a small section at the end of the ArrayList.
        myIndex = myAL.IndexOf( myString, 11 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 11 and the end is at index {1}.", myString, myIndex );
    }

    public static void PrintIndexAndValues(IEnumerable myList)
    {
        int i = 0;
        foreach (Object obj in myList)
            Console.WriteLine("   [{0}]:    {1}", i++, obj);
        Console.WriteLine();
    }

}
/* 
This code produces output similar to the following:

The ArrayList contains the following values:
   [0]:    the
   [1]:    quick
   [2]:    brown
   [3]:    fox
   [4]:    jumps
   [5]:    over
   [6]:    the
   [7]:    lazy
   [8]:    dog
   [9]:    in
   [10]:    the
   [11]:    barn

The first occurrence of "the" is at index 0.
The first occurrence of "the" between index 4 and the end is at index 6.
The first occurrence of "the" between index 6 and index 11 is at index 6.
The first occurrence of "the" between index 11 and the end is at index -1.
*/

---------
        If Me.ddlFPlyEdr.SelectedValue.Trim.Length < 1 Then
            Throw New Exception("[類別]必須輸入")
        Else
            If ("2,3,5").IndexOf(Me.ddlFPlyEdr.SelectedValue.Trim) < 0 Then
                Throw New Exception("[類別]必須選擇2,3或5")
            End If
        End If