Backend(Tests): extra safety measures against null

This is the 3rd and last part of the bugfix for bug 242, which
makes sure that Type.GetType() won't return null again, or if
it does, that we will have enough info (or innerEx with info)
to find culprits and make better decisions later.

src/GWallet.Backend.Tests/MetaMarshalling.fs

@@ -13,8 +13,11 @@ type MetaMarshalling() =
         Assert.That(json, Is.Not.Null)
         Assert.That(json, Is.Not.Empty)
 
-        let wrapperTypeString = Marshalling.ExtractStringType json
-        Assert.That(wrapperTypeString, Is.Not.Null)
-        Assert.That(wrapperTypeString, Is.Not.Empty)
+        let wrapper = Marshalling.ExtractWrapper json
+        Assert.That(wrapper, Is.Not.Null)
+        Assert.That(wrapper.TypeName, Is.Not.Null)
+        Assert.That(wrapper.TypeName, Is.Not.Empty)
+        Assert.That(wrapper.Version, Is.Not.Null)
+        Assert.That(wrapper.Version, Is.Not.Empty)
 
-        Assert.That(wrapperTypeString, Does.Not.Contain "Version=")
+        Assert.That(wrapper.TypeName, Does.Not.Contain "Version=")

src/GWallet.Backend/Marshalling.fs

@@ -145,21 +145,43 @@ module Marshalling =
 
     let private currentVersion = VersionHelper.CURRENT_VERSION
 
-    let ExtractStringType(json: string): string =
+    let ExtractWrapper(json: string): MarshallingWrapper<obj> =
         if String.IsNullOrEmpty json then
             raise <| ArgumentNullException "json"
         let wrapper = JsonConvert.DeserializeObject<MarshallingWrapper<obj>> json
         if Object.ReferenceEquals(wrapper, null) then
             failwith <| SPrintF1 "Failed to extract type from JSON (null check): %s" json
-        wrapper.TypeName
+        if String.IsNullOrEmpty wrapper.TypeName then
+            failwith <| SPrintF1 "Failed to extract type from JSON (inner null check 1): %s" json
+        if String.IsNullOrEmpty wrapper.Version then
+            failwith <| SPrintF1 "Failed to extract type from JSON (inner null check 2): %s" json
+        wrapper
 
     let ExtractType(json: string): Type =
-        let typeName = ExtractStringType json
-        try
-            Type.GetType typeName
-        with
-        | :? NullReferenceException as _nre ->
-            failwith <| SPrintF1 "Failed to extract type from JSON (NRE): %s" json
+        let wrapper = ExtractWrapper json
+
+        let res =
+            try
+                // we prefer an ex with innerException than an NRE caused by the
+                // consumer of this function
+                let throwOnError = true
+
+                Type.GetType(wrapper.TypeName, throwOnError)
+            with
+            | :? NullReferenceException as _nre ->
+                failwith <| SPrintF1 "Failed to extract type from JSON (NRE): %s" json
+            | ex ->
+                let errMsg =
+                    sprintf "Problem when trying to find type '%s' (version '%s')"
+                        wrapper.TypeName
+                        wrapper.Version
+                raise <| Exception(errMsg, ex)
+        if isNull res then
+            failwithf "Could not find type '%s' (version '%s')"
+                wrapper.TypeName
+                wrapper.Version
+        res
+
 
     // FIXME: should we rather use JContainer.Parse? it seems JObject.Parse wouldn't detect error in this: {A:{"B": 1}}
     //        (for more info see replies of https://stackoverflow.com/questions/6903477/need-a-string-json-validator )