c# - How to find index of a substring/character in a string

Find the index of a substring or character in a String
The String represents text as a sequence of UTF-16 code units. The String is a sequential collection of characters that is used to represent text. The String is a sequential collection of System.Char objects.

The following .net c# tutorial code demonstrates how we can find the index of a substring or a character in a String object. So, in this .net c# tutorial code we will find the index position of a specified substring or a specified character within a String instance. Here we will find the index of a substring or a character from a String object using the String IndexOf() method.

The String IndexOf() method reports the zero-based index of the first occurrence of a specified Unicode character or string within this instance. The String IndexOf() method returns -1 if the character or String is not found in this instance.

The String IndexOf(String) method overload reports the zero-based index of the first occurrence of the specified String in this instance. So using this method overload we can find a specified substring’s index position in a String instance. This String IndexOf(String) method returns the index position of the specified substring if it is found in the String instance otherwise it returns -1 if it is not found.

The String IndexOf(Char) method overload reports the zero-based index of the first occurrence of the specified Unicode character in this string. So using this method overload we can find the index position of a specified character from a String instance. This method overload returns the index position of the specified character if it is found in the String instance otherwise it returns -1 if it is not found in the String instance.
string-find-index.aspx

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

<!DOCTYPE html>  
<script runat="server"> 
    protected void Button1_Click(object sender, System.EventArgs e)  
    {
        //this section create a string variable.
        string plants = "Hard Thistle. Cutleaf Toothwort. Leatherleaf Viburnum";

        Label1.Text = "string of plants..................<br />";
        Label1.Text += plants+"<br />";

        string indexToFind = "Cutleaf";
        char indexToFind2 = 'T';

        //this line find/search index of a substring in string
        if (plants.IndexOf(indexToFind) != -1)
        {
            Label1.Text += "<br />Cutleaf found in string index of: ";
            Label1.Text += plants.IndexOf(indexToFind);
        }
        else
        {
            Label1.Text += "<br />Cutleaf not found in string";
        }

        //this line find/search index of a character in string
        if (plants.IndexOf(indexToFind2) != -1)
        {
            Label1.Text += "<br />character 'T' found in string index of: ";
            Label1.Text += plants.IndexOf(indexToFind2);
        }
        else
        {
            Label1.Text += "<br />character 'T' not found in string";
        }
    }  
</script>  
  
<html xmlns="http://www.w3.org/1999/xhtml">  
<head id="Head1" runat="server">  
    <title>c# example - string find index</title>  
</head>  
<body>  
    <form id="form1" runat="server">  
    <div>  
        <h2 style="color:MidnightBlue; font-style:italic;">  
            c# example - string find index
        </h2>  
        <hr width="550" align="left" color="Gainsboro" />  
        <asp:Label   
            ID="Label1"   
            runat="server"  
            Font-Size="Large"
            >  
        </asp:Label>  
        <br /><br />
        <asp:Button   
            ID="Button1"   
            runat="server"   
            Text="string find index"  
            OnClick="Button1_Click"
            Height="40"  
            Font-Bold="true"  
            />  
    </div>  
    </form>  
</body>  
</html>
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