[dotnet] Enable nullability on remote file download APIs

[RenderMichael]

User description

Description

Enable nullability on remote file download APIs, ie. the IHasDownloads interface.

Motivation and Context

Contributes to #14640

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

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PR Type

Enhancement, Bug fix

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Description

• Enabled nullability annotations in IHasDownloads interface and related APIs.

• Improved type safety in RemoteWebDriver methods by handling nullable types.

• Added exception documentation for DownloadFile method.

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Changes walkthrough 📝

	Relevant files
Enhancement	IHasDownloads.cs Enable nullability in `IHasDownloads` interface                    dotnet/src/webdriver/IHasDownloads.cs Enabled nullability annotations for the file. Prepared the interface for nullable reference types. +2/-0
Bug fix	RemoteWebDriver.cs Improve type safety in `RemoteWebDriver` methods                  dotnet/src/webdriver/Remote/RemoteWebDriver.cs Updated GetDownloadableFiles method to handle nullable object arrays. Added exception documentation for DownloadFile method. Improved type safety with nullability handling. +3/-2

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PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

⏱️ Estimated effort to review: 2 🔵🔵⚪⚪⚪

🧪 No relevant tests

🔒 Security concerns Path Traversal: The DownloadFile method accepts a fileName and targetDirectory parameter without validation. This could potentially allow downloading files to unauthorized locations if the parameters contain path traversal sequences (../). Consider adding path sanitization.

⚡ Recommended focus areas for review Null Check The fileName parameter in DownloadFile method should have null check similar to targetDirectory since it's used to download critical files public void DownloadFile(string fileName, string targetDirectory) Type Safety The forced null assertion on value["names"] could throw NullReferenceException if names key doesn't exist in dictionary object?[] namesArray = (object?[])value["names"]!;

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PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
Possible issue	Add defensive null checking Add null check for the 'names' array value from the dictionary to handle cases where the API might return null for this key. dotnet/src/webdriver/Remote/RemoteWebDriver.cs [533-534] -object?[] namesArray = (object?[])value["names"]!; +if (!value.TryGetValue("names", out var namesObj) || namesObj is not object?[] namesArray) +{ + throw new WebDriverException("Missing or invalid 'names' array in response"); +} return namesArray.Select(obj => obj!.ToString()!).ToList(); Apply this suggestion Suggestion importance[1-10]: 8 __ Why: The suggestion adds crucial defensive programming to handle potential null or invalid responses from the API, preventing runtime exceptions and improving error handling with a more descriptive message.	Medium
General	Document null parameter validation Add null check for the 'fileName' parameter in the DownloadFile method, similar to how targetDirectory is documented. Both parameters are required for the operation to succeed. dotnet/src/webdriver/Remote/RemoteWebDriver.cs [540-543] /// <param name="fileName">The name of the file to be downloaded.</param> /// <param name="targetDirectory">The target directory where the file should be downloaded to.</param> -/// <exception cref="ArgumentNullException">If <paramref name="targetDirectory"/> is null.</exception> +/// <exception cref="ArgumentNullException">If <paramref name="fileName"/> or <paramref name="targetDirectory"/> is null.</exception> public void DownloadFile(string fileName, string targetDirectory) Apply this suggestion Suggestion importance[1-10]: 7 __ Why: The suggestion improves API documentation by making null parameter validation explicit for both parameters, which is important for API consumers and consistent with C# best practices.	Medium
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