Added own implementation of IEnumerable.Prepend

IPA.Loader/IPA.Loader.csproj

@@ -131,6 +131,7 @@
     <Compile Include="Utilities\BeatSaber.cs" />
     <Compile Include="Utilities\AlmostVersion.cs" />
     <Compile Include="Utilities\CriticalSection.cs" />
+    <Compile Include="Utilities\EnumerableExtensions.cs" />
     <Compile Include="Utilities\Ref.cs" />
     <Compile Include="Utilities\ReflectionUtil.cs" />
     <Compile Include="Loader\Composite\CompositeIPAPlugin.cs" />

IPA.Loader/Utilities/EnumerableExtensions.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace IPA.Utilities
+{
+    /// <summary>
+    /// Extensions for <see cref="IEnumerable{T}"/> that don't currently exist in <c>System.Linq</c>.
+    /// </summary>
+    public static class EnumerableExtensions
+    {
+        /// <summary>
+        /// Adds a value to the beginning of the sequence.
+        /// </summary>
+        /// <typeparam name="T">the type of the elements of <paramref name="seq"/></typeparam>
+        /// <param name="seq">a sequence of values</param>
+        /// <param name="prep">the value to prepend to <paramref name="seq"/></param>
+        /// <returns>a new sequence beginning with <paramref name="prep"/></returns>
+        public static IEnumerable<T> Prepend<T>(this IEnumerable<T> seq, T prep)
+            => new PrependEnumerable<T>(seq, prep);
+
+        private sealed class PrependEnumerable<T> : IEnumerable<T>
+        {
+            private readonly IEnumerable<T> rest;
+            private readonly T first;
+
+            public PrependEnumerable(IEnumerable<T> rest, T first)
+            {
+                this.rest = rest;
+                this.first = first;
+            }
+
+            public IEnumerator<T> GetEnumerator()
+            {
+                yield return first;
+                foreach (var v in rest) yield return v;
+            }
+
+            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+        }
+    }
+}