/*** Source code ***/
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;

namespace Snippets
{
    public static class IEnumerableExtensions
    {
        /// <summary>
        /// Finds the index in the collection where the predicate evaluates to true.
        /// 
        /// Returns -1 if no matching item found
        /// </summary>
        /// <typeparam name="TSource">Type of collection</typeparam>
        /// <param name="source">Source collection</param>
        /// <param name="predicate">Function to evaluate</param>
        /// <returns>Index where predicate is true, or -1 if not found.</returns>
        public static int IndexWhere<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
        {
            var enumerator = source.GetEnumerator();
            int index = 0;
            while (enumerator.MoveNext())
            {
                TSource obj = enumerator.Current;
                if (predicate(obj))
                    return index;
                index++;
            }
            return -1;
        }

        /// <summary>
        /// Finds the index in the collection where the predicate evaluates to true.
        /// 
        /// Returns -1 if no matching item found
        /// </summary>
        /// <typeparam name="TSource">Type of collection</typeparam>
        /// <param name="source">Source collection</param>
        /// <param name="predicate">Function to evaluate</param>
        /// <returns>Index where predicate is true, or -1 if not found.</returns>
        public static int IndexWhere<TSource>(this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
        {
            var enumerator = source.GetEnumerator();
            int index = 0;
            while (enumerator.MoveNext())
            {
                TSource obj = enumerator.Current;
                if (predicate(obj, index))
                    return index;
                index++;
            }
            return -1;
        }
    }
}

/*** Unit Tests ***/
using ProtectedTrust.Common;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System;
using System.Collections.Generic;

namespace SnippetsTests
{
    
    
    /// <summary>
    ///This is a test class for IEnumerableExtensionsTest and is intended
    ///to contain all IEnumerableExtensionsTest Unit Tests
    ///</summary>
    [TestClass()]
    public class IEnumerableExtensionsTest
    {


        private TestContext testContextInstance;

        /// <summary>
        ///Gets or sets the test context which provides
        ///information about and functionality for the current test run.
        ///</summary>
        public TestContext TestContext
        {
            get
            {
                return testContextInstance;
            }
            set
            {
                testContextInstance = value;
            }
        }

        #region Additional test attributes
        // 
        //You can use the following additional attributes as you write your tests:
        //
        //Use ClassInitialize to run code before running the first test in the class
        //[ClassInitialize()]
        //public static void MyClassInitialize(TestContext testContext)
        //{
        //}
        //
        //Use ClassCleanup to run code after all tests in a class have run
        //[ClassCleanup()]
        //public static void MyClassCleanup()
        //{
        //}
        //
        //Use TestInitialize to run code before running each test
        //[TestInitialize()]
        //public void MyTestInitialize()
        //{
        //}
        //
        //Use TestCleanup to run code after each test has run
        //[TestCleanup()]
        //public void MyTestCleanup()
        //{
        //}
        //
        #endregion


        /// <summary>
        ///A test for IndexWhere
        ///</summary>
        public void IndexWhereTestHelper<TSource>()
        {
            IEnumerable<TSource> source = null; // TODO: Initialize to an appropriate value
            Func<TSource, int, bool> predicate = null; // TODO: Initialize to an appropriate value
            int expected = 0; // TODO: Initialize to an appropriate value
            int actual;
            actual = IEnumerableExtensions.IndexWhere<TSource>(source, predicate);
            Assert.AreEqual(expected, actual);
            Assert.Inconclusive("Verify the correctness of this test method.");
        }

        [TestMethod()]
        public void IndexWhereTest()
        {
            // create list of numbers
            List<int> list = new List<int>();
            for (int i = 0; i < 100; i++)
            {
                list.Add(i);
            }

            // test method with various inputs
            int actual = list.IndexWhere((n) => n == 5);
            Assert.AreEqual(5, actual);

            actual = list.IndexWhere((n) => n == 99);
            Assert.AreEqual(99, actual);

            actual = list.IndexWhere((n) => n == 0);
            Assert.AreEqual(0, actual);

            actual = list.IndexWhere((n) => n == 1);
            Assert.AreEqual(1, actual);

            // check one not found
            actual = list.IndexWhere((n) => n == 23423);
            Assert.AreEqual(-1, actual);
        }

       

        [TestMethod()]
        public void IndexWhereTest1()
        {
            // create list of numbers
            List<int> list = new List<int>();
            for (int i = 0; i < 100; i++)
            {
                list.Add(i);
            }

            // test method with various inputs
            int actual = list.IndexWhere((n, actualRunningIndex) => n == 5);
            Assert.AreEqual(5, actual);

            actual = list.IndexWhere((n, actualRunningIndex) => n == 99);
            Assert.AreEqual(99, actual);

            actual = list.IndexWhere((n, actualRunningIndex) => n == 0);
            Assert.AreEqual(0, actual);

            actual = list.IndexWhere((n, actualRunningIndex) => n == 1);
            Assert.AreEqual(1, actual);

            // check one not found
            actual = list.IndexWhere((n, actualRunningIndex) => n == 23423);
            Assert.AreEqual(-1, actual);
        }

        [TestMethod()]
        public void IndexWhereTestRunningIndex()
        {
            // create list of numbers
            List<int> list = new List<int>();
            for (int i = 0; i < 100; i++)
            {
                list.Add(i);
            }

            int expectedRunningIndex = 0;
            list.IndexWhere((n, actualRunningIndex) => AssertRunningIndex(expectedRunningIndex++, actualRunningIndex));
        }

        private static bool AssertRunningIndex(int expectedRunningIndex, int actualRunningIndex)
        {
            Assert.AreEqual(expectedRunningIndex, actualRunningIndex, "Index passed in predicate method is incorrect.");

            // always return false so the loop doesn't stop
            return false;
        }
    }
}

/*** Source code ***/
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;

namespace SnippetsTests
{
    public static class IEnumerableExtensions
    {
        /// <summary>
        /// Finds the index in the collection where the predicate evaluates to true.
        /// 
        /// Returns -1 if no matching item found
        /// </summary>
        /// <typeparam name="TSource">Type of collection</typeparam>
        /// <param name="source">Source collection</param>
        /// <param name="predicate">Function to evaluate</param>
        /// <returns>Index where predicate is true, or -1 if not found.</returns>
        public static int IndexWhere<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
        {
            var enumerator = source.GetEnumerator();
            int index = 0;
            while (enumerator.MoveNext())
            {
                TSource obj = enumerator.Current;
                if (predicate(obj))
                    return index;
                index++;
            }
            return -1;
        }

        /// <summary>
        /// Finds the index in the collection where the predicate evaluates to true.
        /// 
        /// Returns -1 if no matching item found
        /// </summary>
        /// <typeparam name="TSource">Type of collection</typeparam>
        /// <param name="source">Source collection</param>
        /// <param name="predicate">Function to evaluate</param>
        /// <returns>Index where predicate is true, or -1 if not found.</returns>
        public static int IndexWhere<TSource>(this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
        {
            var enumerator = source.GetEnumerator();
            int index = 0;
            while (enumerator.MoveNext())
            {
                TSource obj = enumerator.Current;
                if (predicate(obj, index))
                    return index;
                index++;
            }
            return -1;
        }
    }
}

/*** Unit Tests ***/
using ProtectedTrust.Common;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System;
using System.Collections.Generic;

namespace ProtectedTrust.Tests.Unit
{
    
    
    /// <summary>
    ///This is a test class for IEnumerableExtensionsTest and is intended
    ///to contain all IEnumerableExtensionsTest Unit Tests
    ///</summary>
    [TestClass()]
    public class IEnumerableExtensionsTest
    {


        private TestContext testContextInstance;

        /// <summary>
        ///Gets or sets the test context which provides
        ///information about and functionality for the current test run.
        ///</summary>
        public TestContext TestContext
        {
            get
            {
                return testContextInstance;
            }
            set
            {
                testContextInstance = value;
            }
        }

        #region Additional test attributes
        // 
        //You can use the following additional attributes as you write your tests:
        //
        //Use ClassInitialize to run code before running the first test in the class
        //[ClassInitialize()]
        //public static void MyClassInitialize(TestContext testContext)
        //{
        //}
        //
        //Use ClassCleanup to run code after all tests in a class have run
        //[ClassCleanup()]
        //public static void MyClassCleanup()
        //{
        //}
        //
        //Use TestInitialize to run code before running each test
        //[TestInitialize()]
        //public void MyTestInitialize()
        //{
        //}
        //
        //Use TestCleanup to run code after each test has run
        //[TestCleanup()]
        //public void MyTestCleanup()
        //{
        //}
        //
        #endregion


        /// <summary>
        ///A test for IndexWhere
        ///</summary>
        public void IndexWhereTestHelper<TSource>()
        {
            IEnumerable<TSource> source = null; // TODO: Initialize to an appropriate value
            Func<TSource, int, bool> predicate = null; // TODO: Initialize to an appropriate value
            int expected = 0; // TODO: Initialize to an appropriate value
            int actual;
            actual = IEnumerableExtensions.IndexWhere<TSource>(source, predicate);
            Assert.AreEqual(expected, actual);
            Assert.Inconclusive("Verify the correctness of this test method.");
        }

        [TestMethod()]
        public void IndexWhereTest()
        {
            // create list of numbers
            List<int> list = new List<int>();
            for (int i = 0; i < 100; i++)
            {
                list.Add(i);
            }

            // test method with various inputs
            int actual = list.IndexWhere((n) => n == 5);
            Assert.AreEqual(5, actual);

            actual = list.IndexWhere((n) => n == 99);
            Assert.AreEqual(99, actual);

            actual = list.IndexWhere((n) => n == 0);
            Assert.AreEqual(0, actual);

            actual = list.IndexWhere((n) => n == 1);
            Assert.AreEqual(1, actual);

            // check one not found
            actual = list.IndexWhere((n) => n == 23423);
            Assert.AreEqual(-1, actual);
        }

       

        [TestMethod()]
        public void IndexWhereTest1()
        {
            // create list of numbers
            List<int> list = new List<int>();
            for (int i = 0; i < 100; i++)
            {
                list.Add(i);
            }

            // test method with various inputs
            int actual = list.IndexWhere((n, actualRunningIndex) => n == 5);
            Assert.AreEqual(5, actual);

            actual = list.IndexWhere((n, actualRunningIndex) => n == 99);
            Assert.AreEqual(99, actual);

            actual = list.IndexWhere((n, actualRunningIndex) => n == 0);
            Assert.AreEqual(0, actual);

            actual = list.IndexWhere((n, actualRunningIndex) => n == 1);
            Assert.AreEqual(1, actual);

            // check one not found
            actual = list.IndexWhere((n, actualRunningIndex) => n == 23423);
            Assert.AreEqual(-1, actual);
        }

        [TestMethod()]
        public void IndexWhereTestRunningIndex()
        {
            // create list of numbers
            List<int> list = new List<int>();
            for (int i = 0; i < 100; i++)
            {
                list.Add(i);
            }

            int expectedRunningIndex = 0;
            list.IndexWhere((n, actualRunningIndex) => AssertRunningIndex(expectedRunningIndex++, actualRunningIndex));
        }

        private static bool AssertRunningIndex(int expectedRunningIndex, int actualRunningIndex)
        {
            Assert.AreEqual(expectedRunningIndex, actualRunningIndex, "Index passed in predicate method is incorrect.");

            // always return false so the loop doesn't stop
            return false;
        }
    }
}

Find the index of an item in a collection with a simple statement.  For example:

int index = myList.IndexWhere(item => item.Something == someOtherThing);

LINQ Index of item - IndexWhere()

C#