List<T>.FindIndex Method (Int32, Predicate<T>)


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Searches for an element that matches the conditions defined by the specified predicate, and returns the zero-based index of the first occurrence within the range of elements in the List<T> that extends from the specified index to the last element.

Namespace:   System.Collections.Generic
Assembly:  mscorlib (in mscorlib.dll)

public int FindIndex(
	int startIndex,
	Predicate<T> match


Type: System.Int32

The zero-based starting index of the search.

Type: System.Predicate<T>

The Predicate<T> delegate that defines the conditions of the element to search for.

Return Value

Type: System.Int32

The zero-based index of the first occurrence of an element that matches the conditions defined by match, if found; otherwise, –1.

Exception Condition

match is null.


startIndex is outside the range of valid indexes for the List<T>.

The List<T> is searched forward starting at startIndex and ending at the last element.

The Predicate<T> is a delegate to a method that returns true if the object passed to it matches the conditions defined in the delegate. The elements of the current List<T> are individually passed to the Predicate<T> delegate. The delegate has the signature:

public bool methodName(T obj)

This method performs a linear search; therefore, this method is an O(n) operation, where n is the number of elements from startIndex to the end of the List<T>.

The following example defines an Employee class with two fields, Name and Id. It also defines an EmployeeSearch class with a single method, StartsWith, that indicates whether the Employee.Name field starts with a specified substring that is supplied to the EmployeeSearch class constructor. Note the signature of this method

public bool StartsWith(Employee e)

corresponds to the signature of the delegate that can be passed to the FindIndex method. The example instantiates a List<Employee> object, adds a number of Employee objets to it, and then calls the FindIndex(Int32, Int32, Predicate<T>) method twice to search the collection starting with its fifth member (that is, the member at index 4). The first time, it searches for the first Employee object whose Name field begins with "J"; the second time, it searches for the first Employee object whose Name field begins with "Ju".

using System;
using System.Collections.Generic;

public class Employee : IComparable
   public String Name { get; set; }
   public int Id { get; set; }

   public int CompareTo(Object o )
      Employee e = o as Employee;
      if (e == null)
         throw new ArgumentException("o is not an Employee object.");

      return Name.CompareTo(e.Name);

public class EmployeeSearch
   String _s;

   public EmployeeSearch(String s)
      _s = s;

   public bool StartsWith(Employee e)
      return e.Name.StartsWith(_s, StringComparison.InvariantCultureIgnoreCase);

public class Example
   public static void Main()
      var employees = new List<Employee>();
      employees.AddRange( new Employee[] { new Employee { Name = "Frank", Id = 2 },
                                           new Employee { Name = "Jill", Id = 3 },
                                           new Employee { Name = "Dave", Id = 5 },
                                           new Employee { Name = "Jack", Id = 8 },
                                           new Employee { Name = "Judith", Id = 12 },
                                           new Employee { Name = "Robert", Id = 14 },
                                           new Employee { Name = "Adam", Id = 1 } } );

      var es = new EmployeeSearch("J");
      int index = employees.FindIndex(4, es.StartsWith);        
      Console.WriteLine("Starting index of'J': {0}",
                        index >= 0 ? index.ToString() : "Not found");

      es = new EmployeeSearch("Ju");
      index = employees.FindIndex(4, es.StartsWith);        
      Console.WriteLine("Starting index of 'Ju': {0}",
                        index >= 0 ? index.ToString() : "Not found");
// The example displays the following output:
//       'J' starts at index 4
//       'Ju' starts at index 5

Universal Windows Platform
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
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