Add Mapper.Reset to reset static mapping

docs/Configuration.md

@@ -217,3 +217,15 @@ Mapper.Configuration.CompileMappings();
 ```
 
 For a few hundred mappings, this may take a couple of seconds.
+
+## Resetting static mapping configuration
+
+The static `Mapper.Initialize` is intended to be called only once. To reset the static mapping configuration (for example, at the start of tests):
+
+```c#
+Mapper.Reset();
+
+Mapper.Initialize(cfg => { /* configure again */ });
+```
+
+`Reset` should not be used in production code. It is intended to support testing scenarios only.

src/AutoMapper/Mapper.cs

@@ -53,6 +53,15 @@ public static void Initialize(MapperConfigurationExpression config)
             Instance = new Mapper(Configuration);
         }
 
+        /// <summary>
+        /// Resets the mapper configuration. Not intended for production use, but for testing scenarios.
+        /// </summary>
+        public static void Reset()
+        {
+            _configuration = null;
+            _instance = null;
+        }
+
         /// <summary>
         /// Execute a mapping from the source object to a new destination object.
         /// The source type is inferred from the source object.

src/UnitTests/StaticMapping.cs

@@ -6,6 +6,9 @@
     using QueryableExtensions;
     using System;
 
+#if !NET40
+    [Collection(nameof(StaticMapping))]
+#endif
     public class StaticMapping
     {
         public class ModelObject
@@ -62,20 +65,26 @@ public class Dest
             public int Value { get; set; }
         }
 
-        static StaticMapping()
+        private void InitializeMapping()
         {
-            Mapper.Initialize(cfg =>
+            lock (this)
             {
-                cfg.CreateMap<Source, Dest>();
-                cfg.CreateMap<ModelObject, ModelDto>();
-                cfg.CreateMap<ModelObject2, ModelDto2>();
-                cfg.CreateMap<ModelObject3, ModelDto3>(MemberList.Source);
-            });
+                Mapper.Reset();
+                Mapper.Initialize(cfg =>
+                {
+                    cfg.CreateMap<Source, Dest>();
+                    cfg.CreateMap<ModelObject, ModelDto>();
+                    cfg.CreateMap<ModelObject2, ModelDto2>();
+                    cfg.CreateMap<ModelObject3, ModelDto3>(MemberList.Source);
+                });
+            }
         }
 
         [Fact]
         public void Can_map_statically()
         {
+            InitializeMapping();
+
             var source = new Source {Value = 5};
 
             var dest = Mapper.Map<Source, Dest>(source);
@@ -86,6 +95,8 @@ public void Can_map_statically()
         [Fact]
         public void Can_project_statically()
         {
+            InitializeMapping();
+
             var source = new Source {Value = 5};
             var sources = new[] {source}.AsQueryable();
 
@@ -98,13 +109,33 @@ public void Can_project_statically()
         [Fact]
         public void Should_fail_a_configuration_check()
         {
+            InitializeMapping();
+
             typeof(AutoMapperConfigurationException).ShouldBeThrownBy(Mapper.AssertConfigurationIsValid);
         }
 
         [Fact]
         public void Should_throw_when_initializing_twice()
         {
-            typeof(InvalidOperationException).ShouldBeThrownBy(() => Mapper.Initialize(_ => { }));
+            typeof(InvalidOperationException).ShouldBeThrownBy(() =>
+            {
+                Mapper.Initialize(_ => { });
+                Mapper.Initialize(_ => { });
+            });
         }
+
+        [Fact]
+        public void Should_not_throw_when_resetting()
+        {
+            var action = new Action(() =>
+            {
+                Mapper.Reset();
+                Mapper.Initialize(cfg => { });
+                Mapper.Reset();
+                Mapper.Initialize(cfg => { });
+            });
+            action.ShouldNotThrow();
+        }
+
     }
 }