c# - How to get the index of an item in a list

Generic List IndexOf() Method

.Net framework generic list IndexOf(T) method search for the specified object and return the zero based index of the first occurrence within the entire List<T>. the IndexOf() method exists in System.Collections.Generic namespace. this IndexOf(T) method require to pass a parameter name 'item'. the 'item' parameter type is T. this 'T' represents the object to locate in the List<T> and the value can be null for reference types.

the method return value data type is System.Int32. return value is a zero based index of the first occurrence of 'item' within the entire List<T>, if the item found. otherwise the method return -1. the method implements as IList<T>.IndexOf(T). if the list contains multiple elements of same name then it return the index of first occurrence.

the following asp.net c# example code demonstrate us how can we get the index number of specific elements in a generic list in an asp.net application.

GenericListIndexOfMethod.aspx


<%@ Page Language="C#" AutoEventWireup="true" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">
    protected void Button1_Click(object sender, System.EventArgs e)
    {
        List<string> colors = new List<string>() {"SaddleBrown","OldLace","DarkBlue"};
        colors.Add("FloralWhite");
        colors.Add("PeachPuff");
        colors.Add("DarkBlue");

        Label1.Text = "List Elements....<br />";
        foreach (string color in colors)
        {
            Label1.Text += "<br />" + color;
        }

        Label1.Text += "<br /><br />IndexOf color 'DarkBlue': " + colors.IndexOf("DarkBlue");
        Label1.Text += "<br /><br />IndexOf color 'PeachPuff': " + colors.IndexOf("PeachPuff");
        Label1.Text += "<br /><br />IndexOf color 'Red': " + colors.IndexOf("Red");
    }
</script>

<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title>Generic List IndexOf() - How to get the specific List element's index</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <h2 style="color:DarkOrchid; font-style:italic;">
            System.Collections.Generic.List IndexOf() Method
            <br /> How to get the specific List element's index
        </h2>
        <hr width="525" align="left" color="Purple" />
        <asp:Label
             ID="Label1"
             runat="server"
             ForeColor="SlateBlue"
             Font-Size="Large"
             Font-Names="Courier New"
             >
        </asp:Label>
        <br /><br />
        <asp:Button 
            ID="Button1"
            runat="server"
            OnClick="Button1_Click"
            Text="Test Generic List IndexOf() Method"
            Height="45"
            Font-Bold="true"
            ForeColor="DodgerBlue"
            />
    </div>
    </form>
</body>
</html>

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