Fix template load failure when template.json contains duplicate JSON property keys

[Copilot]

Problem

Templates (notably MAUI) fail to load with ArgumentException: An item with the same key has already been added. Key: empty on .NET 10+ SDKs. The MAUI template.json has duplicate "empty" keys in the "forms" section. In .NET 10+, JsonObject switched to OrderedDictionary<TKey,TValue> with lazy initialization — JsonObject.Count triggers InitializeDictionary(), which throws on duplicates. JExtensions.PropertiesOf() called jObj.ToList(), and List<T>(ICollection<T>) calls Count to pre-size the list, hitting this path.

Solution

• src/Shared/JExtensions.cs: Replace the bare jObj.ToList() calls in PropertiesOf() with a GetObjectProperties() helper that catches ArgumentException and falls back to GetObjectPropertiesViaDocument(). The fallback serializes via JsonObject.ToJsonString() (safe because WriteTo() uses the underlying JsonElement when _dictionary is null, even after a failed init) and re-enumerates with JsonDocument which tolerates duplicate keys.

// Before
return element is not JsonObject jObj ? [] : jObj.ToList(); // throws on duplicate keys

// After
return element is not JsonObject jObj ? [] : GetObjectProperties(jObj); // handles duplicates gracefully

• test/.../TemplateConfigTests/GenericTests.cs: Two regression tests covering duplicate keys in "forms" and "symbols" sections.

Checks:

• Added unit tests

Original prompt

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This section details on the original issue you should resolve

<issue_title>[NETSDKE2E][Regression]Error: Failed to load template from "C:\xxx\microsoft.maui.templates.net10.10.0.20.nupkg(/content/templates/maui-blazor-solution/.template.config/template.json)" displays when running command in CLI for the first time.</issue_title>
<issue_description>RGRESSION INFO
not repro on VS 18.6 Insiders [11617.261.main] + 11.0.100-preview.3.26166.111 (runtime- 11.0.0-preview.3.26166.111)
not repro in CLI when only installed Insiders VS 18.6 [11618.325.main]

INSTALL STEPS

Insiders VS 18.6 [11618.325.main] + 11.0.100-preview.3.26168.106 (runtime- 11.0.100-preview.3.26168.106) on win-x64

REPRO STEPS
1.In CLI, run the dotnet command for the first time, e.g dotnet new sln

ACTUAL
sln is created successfully,but below error happens

Error: Failed to load template from "C:\Program Files\dotnet\template-packs\microsoft.maui.templates.net10.10.0.20.nupkg(/content/templates/maui-blazor-solution/.template.config/template.json)".
Details: System.ArgumentException: An item with the same key has already been added. Key: empty (Parameter 'key')
at System.Collections.ThrowHelper.ThrowDuplicateKey[TKey](TKey key)
at System.Collections.Generic.OrderedDictionary2.TryInsert(Int32 index, TKey key, TValue value, InsertionBehavior behavior, Int32& keyIndex) at System.Collections.Generic.OrderedDictionary2.Add(TKey key, TValue value)
at System.Text.Json.Nodes.JsonObject.InitializeDictionary()
at System.Text.Json.Nodes.JsonObject.get_Count()
at System.Collections.Generic.List1..ctor(IEnumerable1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source) at Microsoft.TemplateEngine.Orchestrator.RunnableProjects.ConfigModel.TemplateConfigModel..ctor(JsonObject source, ILogger logger, String baselineName) at Microsoft.TemplateEngine.Orchestrator.RunnableProjects.DirectoryBasedTemplate..ctor(IEngineEnvironmentSettings settings, IGenerator generator, IFile templateFile, String baselineName) at Microsoft.TemplateEngine.Orchestrator.RunnableProjects.RunnableProjectGenerator.GetTemplatesFromMountPointInternalAsync(IMountPoint source, CancellationToken cancellationToken) Error: Failed to load template from "C:\Program Files\dotnet\template-packs\microsoft.maui.templates.net10.10.0.20.nupkg(/content/templates/maui-blazor/.template.config/template.json)". Details: System.ArgumentException: An item with the same key has already been added. Key: empty (Parameter 'key') at System.Collections.ThrowHelper.ThrowDuplicateKey[TKey](TKey key) at System.Collections.Generic.OrderedDictionary2.TryInsert(Int32 index, TKey key, TValue value, InsertionBehavior behavior, Int32& keyIndex)
at System.Collections.Generic.OrderedDictionary2.Add(TKey key, TValue value) at System.Text.Json.Nodes.JsonObject.InitializeDictionary() at System.Text.Json.Nodes.JsonObject.get_Count() at System.Collections.Generic.List1..ctor(IEnumerable1 collection) at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source)
at Microsoft.TemplateEngine.Orchestrator.RunnableProjects.ConfigModel.TemplateConfigModel..ctor(JsonObject source, ILogger logger, String baselineName)
at Microsoft.TemplateEngine.Orchestrator.RunnableProjects.DirectoryBasedTemplate..ctor(IEngineEnvironmentSettings settings, IGenerator generator, IFile templateFile, String baselineName)
at Microsoft.TemplateEngine.Orchestrator.RunnableProjects.RunnableProjectGenerator.GetTemplatesFromMountPointInternalAsync(IMountPoint source, CancellationToken cancellationToken)

Expected Result:
sln is created successfully without any erros

Note:
1.We don't meet it in CLI when only installing SDK
2.it not happens when running command in CLI for the second time</issue_description>

<agent_instructions>Can you review the code and determine if this is something hte template engine should fix or something in the authoring of the maui templates? From the description, it looks like an issue if you install a newer sdk and older SDK SxS and the new SDK finds a duplicate template between the new and old SDKS.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@balachir @marcpopMSFT is this an issue in the .NET SDK that needs to be looked at for .NET 11 P3? From [Nicole's scenario above](https://github.com//issues/10047#issuecomment-4128056053), this reproduces when only .NET SDK is installed and even the install of VS is not necessary. @marcpopMSFT Moving to templating because that's where the callstack is from.

• Fixes [NETSDKE2E][Regression]Error: Failed to load template from "C:\xxx\microsoft.maui.templates.net10.10.0.20.nupkg(/content/templates/maui-blazor-solution/.template.config/template.json)" displays when running command in CLI for the first time. #10047

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[marcpopMSFT]

The original issue was a case collision to we'll prefer https://github.com/dotnet/templating/pull/10049/changes