{"id":2490,"date":"2023-07-01T07:53:43","date_gmt":"2023-07-01T00:53:43","guid":{"rendered":"https:\/\/csharptutorial.net\/?page_id=2490"},"modified":"2023-07-01T09:46:09","modified_gmt":"2023-07-01T02:46:09","slug":"csharp-string-indexof","status":"publish","type":"page","link":"https:\/\/www.csharptutorial.net\/csharp-string-methods\/csharp-string-indexof\/","title":{"rendered":"C# String IndexOf"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you will learn how to use the C# String IndexOf()<\/code> method to return the position of the first occurrence of a specified string within the current string.<\/p>\n\n\n\n

Introduction to the C# String IndexOf() method<\/h2>\n\n\n\n

The String IndexOf()<\/code> method allows you to check whether a string is present in the current string:<\/p>\n\n\n

public<\/span> int<\/span> IndexOf<\/span> (string<\/span> value<\/span><\/span>)<\/span>;<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
The IndexOf()<\/code> method accepts a parameter value<\/code> that is a string to search for. It returns the position of the first occurrence of the string value<\/code> in the current string. <\/p>\n\n\n\n
If the string value<\/code> is not present in the current string, the IndexOf()<\/code> method returns -1<\/code>.<\/p>\n\n\n\n
To determine if a string exists within another string without retrieving its position, you can use the Contains()<\/a><\/code> method instead.<\/p>\n\n\n\n
The following example uses the IndexOf()<\/code> method to get the starting position of the string \"is\"<\/code> in the string \"C# is awesome\"<\/code>:<\/p>\n\n\n
using<\/span> static<\/span> System.Console;\n\nvar<\/span> message = \"C# is awesome\"<\/span>;\nvar<\/span> index = message.IndexOf(\"is\"<\/span>);\nWriteLine(index);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
3<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
By default, the IndexOf()<\/code> method uses the ordinal comparison<\/strong> which considers both case sensitivity<\/strong> and culture insensitivity<\/strong> for string comparison.<\/p>\n\n\n\n
For example, the following program outputs -1<\/code> because the string \"IS\"<\/code> which is the uppercase of the string \"is\"<\/code>, is not present in the string \"C# is Awesome\"<\/code>:<\/p>\n\n\n
using<\/span> static<\/span> System.Console;\n\nvar<\/span> message = \"C# is awesome\"<\/span>;\nvar<\/span> index = message.IndexOf(\"IS\"<\/span>);\nWriteLine(index);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
-1<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
To compare strings case-sensitively, you use the following overload of the IndexOf()<\/code> method:<\/p>\n\n\n
public<\/span> int<\/span> IndexOf<\/span> (\n   string<\/span> value<\/span>, \n   StringComparison comparisonType\n<\/span>)<\/span>;<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this method, the comparisonType<\/code> is one of the values of the StringComparison<\/code> enum. <\/p>\n\n\n\n
For example, the following uses the StringComparison.OrdinalIgnoreCase<\/code> that ignores cases of the strings when comparing them:<\/p>\n\n\n
using<\/span> static<\/span> System.Console;\n\nvar<\/span> message = \"C# is awesome\"<\/span>;\nvar<\/span> index = message.IndexOf(\"IS\"<\/span>, StringComparison.OrdinalIgnoreCase);\nWriteLine(index);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
3<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
By default, the IndexOf()<\/code> method starts searching for a substring from the beginning of a string. To start searching for a substring at a specified character position, you use the following overload of the IndexOf()<\/code> method:<\/p>\n\n\n
public<\/span> int<\/span> IndexOf<\/span> (\n   char<\/span> value<\/span>, \n   int<\/span> startIndex\n<\/span>)<\/span>;<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this syntax, the startIndex<\/code> specifies the starting index to search for the string value in the current string. <\/p>\n\n\n\n
For example, the following use the IndexOf()<\/code> method to find the index of the second position of the string \"se\"<\/code> in the string \"She sees the sea\"<\/code>:<\/p>\n\n\n
using<\/span> static<\/span> System.Console;\n\nvar<\/span> message = \"She sees the sea\"<\/span>;\nvar<\/span> index = message.IndexOf(\"se\"<\/span>);\n\nif<\/span>(index != -1<\/span>)\n{\n    var<\/span> secondIndex = message.IndexOf(\"se\"<\/span>, index + 1<\/span>);\n    WriteLine(secondIndex);\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
13<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
To search for a string within a substring at a specified index within a number of character positions, you use the following overload of the IndexOf() method:<\/p>\n\n\n
public<\/span> int<\/span> IndexOf<\/span> (\n    string<\/span> value<\/span>, \n    int<\/span> startIndex, \n    int<\/span> count\n<\/span>)<\/span>;<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this syntax, the count<\/code> is the number of characters positions to search.<\/p>\n\n\n\n
The following example uses the count<\/code> parameter to search for the second occurrence of the string \"se\"<\/code> in the string \"She sees the sea\"<\/code> within 5 character positions:<\/p>\n\n\n
using<\/span> static<\/span> System.Console;\n\nvar<\/span> message = \"She sees the sea\"<\/span>;\nvar<\/span> index = message.IndexOf(\"se\"<\/span>);\n\nif<\/span> (index != -1<\/span>)\n{\n    var<\/span> secondIndex = message.IndexOf(\"se\"<\/span>, index + 1<\/span>, 5<\/span>);\n    WriteLine(secondIndex);\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
-1<\/code><\/span><\/pre>\n\n\n
The program output -1<\/code> because within 5 character positions from the index+1<\/code>, there is no occurrence of the string \"se\"<\/code> in the searched string.<\/p>\n\n\n\n
Summary<\/h2>\n\n\n\n
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  • Use C# String IndexOf()<\/code> method to find the zero-based index of the first occurrence of a string within the current string.<\/li>\n<\/ul>\n
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