How to convert string value to generic type parameter? [duplicate]

Question

How can I pass a generic method type parameter as a variable to it? Below given is the crud example of what I want to achieve. It's just for a demo purpose, not the actual code. I can use if...else or switch to go through passed category and call generic method with corresponding type parameter.

[HttpGet]
[Route("{category}")]
public IActionResult Get(string category)
{
    object data = new object();
    string json = GetData(category);
    if (category == "User")
    {
        data = JsonConvert.DeserializeObject<User>(json);
    }
    else if (category == "Organization")
    {
        data = JsonConvert.DeserializeObject<Organization>(json);
    }

    return Ok(data);
}

Assume GetData is a function which gives me a collection in a JSON format based on passed category. I want to call a generic method, DeserializeObject<T>(...), which requires a type parameter. The class which refers the type parameter is related to passed category. How to achieve like given below?

[HttpGet]
[Route("{category}")]
public IActionResult Get(string category)
{
    string json = GetData(category);
    T categoryType = GetCategoryType(category); // what should be here???
    object data = JsonConvert.DeserializeObject<categoryType>(json);

    return Ok(data);
}

"How can I pass a generic method type parameter as a variable to it?" - that isn't how generics are meant to be used. Generics enable static (i.e. compile-time) type information to be reified - for this scenario you should stick to good ol' fashioned vtable polymorphism. — Dai
– Dai, Commented Jul 14, 2021 at 13:53
A good counter-example to your notion is Get( category: DeleteEntireFileSystem ). You should never let external/remote clients have full control over deserialization: it leads to far too many vulnerabilities. — Dai
– Dai, Commented Jul 14, 2021 at 13:54
Does this answer your question? Setting generic type at runtime — Franz Gleichmann
– Franz Gleichmann, Commented Jul 14, 2021 at 13:55
"which gives me a collection in a JSON format based on passed category" - I'll bet you $50 USD that this is not what it does - simply because if you call GetData( "nextweekslotterynumbers" ) you won't get a JSON array of 7 numbers 0-49. You need to find out exactly what data GetData can return and then enforce that restriction in your controller action. This is part of defence in depth. — Dai
– Dai, Commented Jul 14, 2021 at 13:56
@Dai please don't focus on what GetData function is doing. It was taken just as an example. — Farhan Ghumra
– Farhan Ghumra, Commented Jul 14, 2021 at 14:12

Charlieface · Accepted Answer · 2021-07-14 14:11:16Z

1

JsonConvert.DeserializeObject does not require generics, it also accepts a Type object. Generics is not the right solution here.

Instead use this overload

JsonConvert.DeserializeObject(json, Type.GetType(yourNamespace + category))

I strongly suggest you maintain a whitelist of possible types if you are receiving the type name from user input.

answered Jul 14, 2021 at 14:11

Charlieface

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2 Comments

Farhan Ghumra Over a year ago

Here question is about any generic function not for Json.NET's DeserializeObject function. I mentioned it as an example just.

Charlieface Over a year ago

Then the duplicate is whta you want. If it was just an example of some generic code then you should have made that clear. I have answered the question that was asked

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