fix: clearer error when partial registered on wrong Handlebars instance (issue #545)

source/Handlebars.Test/IssueTests.cs

@@ -726,6 +726,37 @@ public void UnrecognisedExpressionThrowsOutOfMemoryException()
             Assert.Throws<HandlebarsCompilerException>(()=> Handlebars.Compile(source));
         }
 
+        // Issue: https://github.com/Handlebars-Net/Handlebars.Net/issues/545
+        // Partial registered on an instance should be found during render on that same instance
+        [Fact]
+        public void Issue545_PartialRegisteredOnInstanceIsFoundDuringRender()
+        {
+            // This is the CORRECT pattern — register and compile on same instance
+            var handlebars = Handlebars.Create();
+            handlebars.RegisterTemplate("content", "<p>{{message}}</p>");
+            var template = handlebars.Compile("<div>{{> content}}</div>");
+            var result = template(new { message = "Hello" });
+            Assert.Equal("<div><p>Hello</p></div>", result);
+        }
+
+        // Issue: https://github.com/Handlebars-Net/Handlebars.Net/issues/545
+        // Mixing static registration with instance compilation should give a helpful error message
+        [Fact]
+        public void Issue545_MixingStaticAndInstanceGivesHelpfulError()
+        {
+            // Register on static instance
+            Handlebars.RegisterTemplate("staticContent545", "<p>static</p>");
+
+            // Compile on a different instance — should give a useful error
+            var handlebars = Handlebars.Create();
+            var template = handlebars.Compile("<div>{{> staticContent545}}</div>");
+            var ex = Assert.Throws<HandlebarsRuntimeException>(() => template(new { }));
+            // The error should mention the partial name clearly
+            Assert.Contains("staticContent545", ex.Message);
+            // The error should hint that the user may have registered on the wrong instance
+            Assert.Contains("static Handlebars class", ex.Message);
+        }
+
         // Issue: https://github.com/Handlebars-Net/Handlebars.Net/issues/521
         // Hashtable with an uppercase key should be accessible using the same-cased expression.
         // The IDictionary accessor was incorrectly lowercasing the lookup key.

source/Handlebars.Test/PartialTests.cs

@@ -680,7 +680,7 @@ public void RecursionUnboundedBlockPartialWithSpecialNamedPartial()
             ex = Assert.IsType<HandlebarsRuntimeException>(ex.InnerException);
             Assert.Equal("Runtime error while rendering partial 'myPartial', see inner exception for more information", ex.Message);
             ex = Assert.IsType<HandlebarsRuntimeException>(ex.InnerException);
-            Assert.Equal("Referenced partial name @partial-block could not be resolved", ex.Message);
+            Assert.StartsWith("Referenced partial name '@partial-block' could not be resolved", ex.Message);
             Assert.Null(ex.InnerException);
         }
 
@@ -694,7 +694,7 @@ public void TemplateWithSpecialNamedPartial()
             var data = new {};
 
             var ex = Assert.Throws<HandlebarsRuntimeException>(() => template(data));
-            Assert.Equal("Referenced partial name @partial-block could not be resolved", ex.Message);
+            Assert.StartsWith("Referenced partial name '@partial-block' could not be resolved", ex.Message);
         }
 
         public class TestMissingPartialTemplateHandler : IMissingPartialTemplateHandler

source/Handlebars/Compiler/Translation/Expression/PartialBinder.cs

@@ -112,8 +112,8 @@ private static void InvokePartialWithFallback(
             if (context.PartialBlockTemplate == null)
             {
                 if (configuration.MissingPartialTemplateHandler == null)
-                    throw new HandlebarsRuntimeException($"Referenced partial name {partialName} could not be resolved");
-
+                    throw new HandlebarsRuntimeException($"Referenced partial name '{partialName}' could not be resolved. If you registered the partial on the static Handlebars class, make sure to also compile and render using the same static class (or register the partial on the IHandlebars instance you are using to compile).");
+                
                 configuration.MissingPartialTemplateHandler.Handle(configuration, partialName, writer);
                 return;
             }