A minimum tactical fix for issue #697 (#698)

* Provoke the issue

* Minimum fix

(cherry picked from commit 0f23047)

* Revert "Provoke the issue"

This reverts commit 1a2275b.

* Add unit test

(cherry picked from commit 185ba88)

* Use existing infrastructure

* Quick fix

Co-authored-by: Jb Evain <[email protected]>

Mono.Cecil.Cil/MethodBody.cs

@@ -401,15 +401,23 @@ InstructionOffset ResolveInstructionOffset(InstructionOffset inputOffset, ref In
 				for (int i = cache.Index; i < items.Length; i++) {
 					cache.Index = i;
 					cache.Offset = size;
-					cache.Instruction = items [i];
+
+					var item = items [i];
+
+					// Allow for trailing null values in the case of
+					// instructions.Size < instructions.Capacity
+					if (item == null)
+						break;
+
+					cache.Instruction = item;
 
 					if (cache.Offset == offset)
 						return new InstructionOffset (cache.Instruction);
 
 					if (cache.Offset > offset)
 						return new InstructionOffset (items [i - 1]);
 
-					size += items [i].GetSize ();
+					size += item.GetSize ();
 				}
 
 				return new InstructionOffset ();

Test/Mono.Cecil.Tests/ILProcessorTests.cs

@@ -5,6 +5,7 @@
 
 using Mono.Cecil;
 using Mono.Cecil.Cil;
+using Mono.Cecil.Mdb;
 using Mono.Cecil.Pdb;
 using NUnit.Framework;
 
@@ -40,6 +41,28 @@ public void InsertBefore ()
 			AssertOpCodeSequence (new [] { OpCodes.Ldloc_0, OpCodes.Ldloc_1, OpCodes.Ldloc_2, OpCodes.Ldloc_3 }, method);
 		}
 
+		[Test]
+		public void InsertBeforeIssue697 ()
+		{
+			var parameters = new ReaderParameters { SymbolReaderProvider = new MdbReaderProvider () };
+			using (var module = GetResourceModule ("Issue697.dll", parameters))
+			{
+				var pathGetterDef = module.GetTypes ()
+					.SelectMany (t => t.Methods)
+					.First (m => m.Name.Equals ("get_Defer"));
+
+				var body = pathGetterDef.Body;
+				var worker = body.GetILProcessor ();
+				var initialBody = body.Instructions.ToList ();
+				var head = initialBody.First ();
+				var opcode = worker.Create (OpCodes.Ldc_I4_1);
+				worker.InsertBefore (head, opcode);
+				worker.InsertBefore (head, worker.Create (OpCodes.Ret));
+				initialBody.ForEach (worker.Remove);
+				AssertOpCodeSequence (new [] { OpCodes.Ldc_I4_1, OpCodes.Ret }, pathGetterDef.body);	
+			}
+		}
+
 		[Test]
 		public void InsertAfter ()
 		{