diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9a82133..dfdf9a8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,7 @@
 - X10D: Added `IEnumerable<T>.WhereNot(Func<T, bool>)`
 - X10D: Added `IList<T>.RemoveRange(Range)`
 - X10D: Added `IList<T>.Swap(IList<T>)` (#62)
+- X10D: Added `IReadOnlyList<T>.IndexOf(T[, int[, int]])`
 - X10D: Added `Point.IsOnLine(LineF)`, `Point.IsOnLine(PointF, PointF)`, and `Point.IsOnLine(Vector2, Vector2)`
 - X10D: Added `PointF.IsOnLine(LineF)`, `PointF.IsOnLine(PointF, PointF)`, and `PointF.IsOnLine(Vector2, Vector2)`
 - X10D: Added `Point.ToSize()`
diff --git a/X10D.Tests/src/Collections/ListTests.cs b/X10D.Tests/src/Collections/ListTests.cs
index dc6574a..2f17998 100644
--- a/X10D.Tests/src/Collections/ListTests.cs
+++ b/X10D.Tests/src/Collections/ListTests.cs
@@ -79,6 +79,67 @@ public class ListTests
         Assert.ThrowsException<ArgumentNullException>(() => list!.Fill(0, 0, 0));
     }
 
+    [TestMethod]
+    public void IndexOf_ShouldReturnCorrectValue_FromStartOfList()
+    {
+        int[] array = {0, 1, 2, 3, 4};
+        Assert.AreEqual(2, array.IndexOf(2));
+        Assert.AreEqual(2, array.IndexOf(2, 0));
+        Assert.AreEqual(2, array.IndexOf(2, 0, 5));
+    }
+
+    [TestMethod]
+    public void IndexOf_ShouldReturnCorrectValue_GivenSubRange()
+    {
+        int[] array = {0, 1, 2, 3, 4, 0};
+        Assert.AreEqual(0, array.IndexOf(0));
+        Assert.AreEqual(0, array.IndexOf(0, 0));
+        Assert.AreEqual(0, array.IndexOf(0, 0, 5));
+
+        Assert.AreEqual(5, array.IndexOf(0, 1));
+        Assert.AreEqual(5, array.IndexOf(0, 1, 5));
+    }
+
+    [TestMethod]
+    public void IndexOf_ShouldReturnNegative1_ForEmptyList()
+    {
+        int[] array = Array.Empty<int>();
+        Assert.AreEqual(-1, array.IndexOf(0));
+        Assert.AreEqual(-1, array.IndexOf(0, 0));
+        Assert.AreEqual(-1, array.IndexOf(0, 0, 0));
+    }
+
+    [TestMethod]
+    public void IndexOf_ShouldThrowArgumentNullException_GivenNullList()
+    {
+        int[]? array = null;
+        Assert.ThrowsException<ArgumentNullException>(() => array!.IndexOf(0));
+        Assert.ThrowsException<ArgumentNullException>(() => array!.IndexOf(0, 0));
+        Assert.ThrowsException<ArgumentNullException>(() => array!.IndexOf(0, 0, 0));
+    }
+
+    [TestMethod]
+    public void IndexOf_ShouldThrowArgumentOutOfRangeException_GivenNegativeCount()
+    {
+        int[] array = Array.Empty<int>();
+        Assert.ThrowsException<ArgumentOutOfRangeException>(() => array.IndexOf(0, 0, -1));
+    }
+
+    [TestMethod]
+    public void IndexOf_ShouldThrowArgumentOutOfRangeException_GivenNegativeStartIndex()
+    {
+        int[] array = Array.Empty<int>();
+        Assert.ThrowsException<ArgumentOutOfRangeException>(() => array.IndexOf(0, -1));
+        Assert.ThrowsException<ArgumentOutOfRangeException>(() => array.IndexOf(0, -1, 0));
+    }
+
+    [TestMethod]
+    public void IndexOf_ShouldThrowArgumentOutOfRangeException_GivenInvalidStartIndexCountPair()
+    {
+        int[] array = {0, 1, 2};
+        Assert.ThrowsException<ArgumentOutOfRangeException>(() => array.IndexOf(0, 2, 4));
+    }
+
     [TestMethod]
     public void Random_ShouldReturnContainedObject_GivenNotNull()
     {
diff --git a/X10D/src/Collections/ListExtensions.cs b/X10D/src/Collections/ListExtensions.cs
index 3d35aed..086b70e 100644
--- a/X10D/src/Collections/ListExtensions.cs
+++ b/X10D/src/Collections/ListExtensions.cs
@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using System.Diagnostics.Contracts;
 using X10D.Core;
 
@@ -88,6 +89,131 @@ public static class ListExtensions
         }
     }
 
+    /// <summary>
+    ///     Searches for the specified object and returns the zero-based index of the first occurrence within the entire
+    ///     <see cref="IReadOnlyList{T}" />.
+    /// </summary>
+    /// <param name="source">The list to search</param>
+    /// <param name="item">
+    ///     The object to locate in the <see cref="IReadOnlyList{T}" />. The value can be <see langword="true" /> for reference
+    ///     types.
+    /// </param>
+    /// <typeparam name="T">The type of elements in <paramref name="source" />.</typeparam>
+    /// <returns>
+    ///     The zero-based index of the first occurrence of item within the entire <see cref="List{T}" />, if found; otherwise,
+    ///     -1.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
+    public static int IndexOf<T>(this IReadOnlyList<T?> source, T? item)
+    {
+#if NET6_0_OR_GREATER
+        ArgumentNullException.ThrowIfNull(source);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+#endif
+
+        return source.IndexOf(item, 0, source.Count);
+    }
+
+    /// <summary>
+    ///     Searches for the specified object and returns the zero-based index of the first occurrence within the range of
+    ///     elements in the <see cref="IReadOnlyList{T}" /> that extends from the specified index to the last element.
+    /// </summary>
+    /// <param name="source">The list to search</param>
+    /// <param name="item">
+    ///     The object to locate in the <see cref="IReadOnlyList{T}" />. The value can be <see langword="true" /> for reference
+    ///     types.
+    /// </param>
+    /// <param name="startIndex">The zero-based starting index of the search. 0 (zero) is valid in an empty list.</param>
+    /// <typeparam name="T">The type of elements in <paramref name="source" />.</typeparam>
+    /// <returns>
+    ///     The zero-based index of the first occurrence of item within the range of elements in the
+    ///     <see cref="IReadOnlyList{T}" /> that starts at index and contains count number of elements, if found; otherwise, -1.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
+    /// <exception cref="ArgumentOutOfRangeException">
+    ///     <paramref name="startIndex" /> is outside the range of valid indexes for the <see cref="IReadOnlyList{T}" />.
+    /// </exception>
+    public static int IndexOf<T>(this IReadOnlyList<T?> source, T? item, int startIndex)
+    {
+#if NET6_0_OR_GREATER
+        ArgumentNullException.ThrowIfNull(source);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+#endif
+
+        return source.IndexOf(item, startIndex, source.Count - startIndex);
+    }
+
+    /// <summary>
+    ///     Searches for the specified object and returns the zero-based index of the first occurrence within the range of
+    ///     elements in the <see cref="IReadOnlyList{T}" /> that starts at the specified index and contains the specified number
+    ///     of elements.
+    /// </summary>
+    /// <param name="source">The list to search</param>
+    /// <param name="item">
+    ///     The object to locate in the <see cref="IReadOnlyList{T}" />. The value can be <see langword="true" /> for reference
+    ///     types.
+    /// </param>
+    /// <param name="startIndex">The zero-based starting index of the search. 0 (zero) is valid in an empty list.</param>
+    /// <param name="count">The number of elements in the section to search.</param>
+    /// <typeparam name="T">The type of elements in <paramref name="source" />.</typeparam>
+    /// <returns>
+    ///     The zero-based index of the first occurrence of item within the range of elements in the
+    ///     <see cref="IReadOnlyList{T}" /> that starts at index and contains count number of elements, if found; otherwise, -1.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
+    /// <exception cref="ArgumentOutOfRangeException">
+    ///     <para>
+    ///         <paramref name="startIndex" /> is outside the range of valid indexes for the <see cref="IReadOnlyList{T}" />.
+    ///     </para>
+    ///     -or-
+    ///     <para><paramref name="count" /> is less than 0.</para>
+    ///     -or-
+    ///     <para>
+    ///         <paramref name="startIndex" /> and <paramref name="count" /> do not specify a valid section in the
+    ///         <see cref="IReadOnlyList{T}" />.
+    ///     </para>
+    /// </exception>
+    public static int IndexOf<T>(this IReadOnlyList<T?> source, T? item, int startIndex, int count)
+    {
+#if NET6_0_OR_GREATER
+        ArgumentNullException.ThrowIfNull(source);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+#endif
+
+        if (startIndex < 0 || startIndex > source.Count)
+        {
+            throw new ArgumentOutOfRangeException(nameof(startIndex), ExceptionMessages.IndexOutOfRange);
+        }
+
+        if (count < 0 || count > source.Count - startIndex)
+        {
+            throw new ArgumentOutOfRangeException(nameof(count), ExceptionMessages.CountMustBeInRange);
+        }
+
+        int endIndex = startIndex + count;
+        for (int index = startIndex; index < endIndex; index++)
+        {
+            if (EqualityComparer<T>.Default.Equals(source[index]!, item!))
+            {
+                return index;
+            }
+        }
+
+        return -1;
+    }
+
     /// <summary>
     ///     Returns a random element from the current list using a specified <see cref="System.Random" /> instance.
     /// </summary>
diff --git a/X10D/src/ExceptionMessages.Designer.cs b/X10D/src/ExceptionMessages.Designer.cs
index 7dd2526..8e7801e 100644
--- a/X10D/src/ExceptionMessages.Designer.cs
+++ b/X10D/src/ExceptionMessages.Designer.cs
@@ -1,7 +1,6 @@
 //------------------------------------------------------------------------------
 // <auto-generated>
 //     This code was generated by a tool.
-//     Runtime Version:4.0.30319.42000
 //
 //     Changes to this file may cause incorrect behavior and will be lost if
 //     the code is regenerated.
@@ -78,6 +77,15 @@ namespace X10D {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Count must be positive and count must refer to a location within the string/array/collection..
+        /// </summary>
+        internal static string CountMustBeInRange {
+            get {
+                return ResourceManager.GetString("CountMustBeInRange", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to The end index must be less than the list count..
         /// </summary>
@@ -114,6 +122,15 @@ namespace X10D {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Index was out of range. Must be non-negative and less than or equal to the size of the collection..
+        /// </summary>
+        internal static string IndexOutOfRange {
+            get {
+                return ResourceManager.GetString("IndexOutOfRange", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Length must be greater than or equal to 0..
         /// </summary>
diff --git a/X10D/src/ExceptionMessages.resx b/X10D/src/ExceptionMessages.resx
index c2bbd2e..b2a13ce 100644
--- a/X10D/src/ExceptionMessages.resx
+++ b/X10D/src/ExceptionMessages.resx
@@ -26,6 +26,9 @@
     <data name="BufferTooSmall" xml:space="preserve">
         <value>The buffer is too small to contain the data.</value>
     </data>
+    <data name="CountMustBeInRange" xml:space="preserve">
+        <value>Count must be positive and count must refer to a location within the string/array/collection.</value>
+    </data>
     <data name="EndIndexLessThanStartIndex" xml:space="preserve">
         <value>The end index must be greater than or equal to the start index.</value>
     </data>
@@ -47,6 +50,9 @@
     <data name="HashAlgorithmCreateReturnedNull" xml:space="preserve">
         <value>HashAlgorithm's Create method returned null reference.</value>
     </data>
+    <data name="IndexOutOfRange" xml:space="preserve">
+        <value>Index was out of range. Must be non-negative and less than or equal to the size of the collection.</value>
+    </data>
     <data name="LengthGreaterThanOrEqualTo0" xml:space="preserve">
         <value>Length must be greater than or equal to 0.</value>
     </data>