Throw Exception In Content.Load When assetName Is A Rooted Path. (M…

…onoGame#8569)

Fixes MonoGame#8568 


### Description of Change
Adds a `Path.IsRootedPath` check on the `assetName` parameter, and if it
is a rooted path, throws a `ContentLoadException` with a clear exception
message on why the exception occurred.

Previous behavior would have allowed a rooted path as the `assetName`
parameter which would later throw a `FileNotFoundException` in the
`ContentManager.OpenStream` method when attempting to open the stream.
This is due to how `Path.Combine` works, where if the second parameter
is a rooted path, it ignores the first parameter.

While technically the `FileNotFoundException` is correct when the stream
is attempted to open, the exception doesn't clearly describe to the
developer why it occurred. The `assetName` parameter is defined as being
a relative path, so if it is a rooted path, the exception should be
thrown earlier to tell the developer they used a rooted path and need to
use a relative path instead.
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MonoGame.Framework/Content/ContentManager.cs

@@ -345,6 +345,10 @@ public virtual T Load<T>(string assetName)
             {
                 throw new ObjectDisposedException("ContentManager");
             }
+            if(Path.IsPathRooted(assetName))
+            {
+                throw new ContentLoadException("assetName '" + assetName + "' cannot be a rooted (absolute) path. Remove any leading drive letters (e.g. 'C:'), forward slashes or backslashes");
+            }
 
             T result = default(T);
 

Tests/Framework/Content/ContentManagerTest.cs

@@ -0,0 +1,21 @@
+using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Content;
+using Microsoft.Xna.Framework.Graphics;
+using NUnit.Framework;
+
+namespace MonoGame.Tests.Content
+{
+    public class ContentManagerTest
+    {
+        [TestCase("C:\\image.png")]
+        [TestCase("\\image.png")]
+        [TestCase("/image.png")]
+        public void ThrowExceptionIfAssetNameIsRootedPath(string assetName)
+        {
+            GameServiceContainer services = new GameServiceContainer();
+            ContentManager content = new ContentManager(services, "Content");
+            var exception = Assert.Throws<ContentLoadException>(() => content.Load<Texture2D>(assetName));
+            StringAssert.Contains("rooted (absolute)", exception.Message);
+        }
+    }
+}