Cannot implicitly convert type 'task<ienumerable<producttypesdata>>' to 'task<actionresult<ienumerable<producttypesdata>>>' [API]

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Hi All,

I am facing a scenario where I am trying to return a list of "ProductTypes". I may need this list in many places in the code and so defined an async method and then calling that method from another async [HttpGet] method and trying to retrun the resut to the frontend.

I can understand we cannot convert an Interface to another Interface, but since am returning a list of ProductTypes, so it shouldn't have been any problem.

The async method is shown below:

<pre><pre>private async Task<IEnumerable<ProductTypesData>> GetProductTypes() {
            return await _context.ProductTypePool
                .Where(ProdType => ProdType.ProdParentTypeId == 0 && ProdType.IsActive == true)
                .OrderBy(ProdType => ProdType.ProdTypeId)
                .Select(ProdType => new ProductTypesData { ProdTypeId = ProdType.ProdTypeId, ProdTypeName = ProdType.ProdTypeName, ProdTypeDesc = ProdType.ProdTypeDesc})
                .ToListAsync();
        }

Then, am calling this method from annother async [HttpGet] method, as shown below:

[HttpGet]
        public async Task<ActionResult<IEnumerable<ProductTypesData>>> GetAllProductTypes()
        {
            return this.GetProductTypes();
        }

What I have tried:

I tried to wrapthe result into a concrete List class:

[HttpGet]
        public async Task<ActionResult<IEnumerable<ProductTypesData>>> GetAllProductTypes()
        {
            return new List<ProductTypesData>(this.GetProductTypes());
        }

But, this didn't work out.

I also tried to Use IList instead of IEnumerable but things are same there also.

Please help me knowing the way out in such conditions where I may have to return a list of data and that list may be used in many places in the code.

Thanks.

Posted 6-Feb-23 21:20pm

Agnib Pyne 2022

Updated 6-Feb-23 22:19pm

v2

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Comments

Graeme_Grant 7-Feb-23 4:09am

you're returning IEnumerable<ProductTypesData> not ActionResult<IEnumerable<ProductTypesData>>

Agnib Pyne 2022 7-Feb-23 4:32am

Yes. Can I mark the async method (GetProductTypes()) as Task<actionresult<ienumerable>> too? If not, then can I mark [HttpGet] method as Task<ienumerable<...>> only since am not using any HttpResponseMessage within the [HttpGet] method. I am just returning the data to the UI and dispaying it.

2 solutions

Solution 2

public async Task<actionresult<ienumerable<state>>> GetStates()
{
return await _context.States.ToListAsync();
}

Posted 18hrs ago

Member 16249372

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Richard Deeming · Accepted Answer · 2023-02-06T22:19:00

Solution 1

You need to await the task returned from GetProductTypes.

Assuming you're using .NET Core / .NET 5+, you can then simply return that result - ActionResult<T> has an implicit conversion[^] from T:

C#

[HttpGet] public async Task<ActionResult<IEnumerable<ProductTypesData>>> GetAllProductTypes() { return await this.GetProductTypes(); }

Edit: Implicit conversions don't work for interfaces. You can either change the return type of the GetProductTypes method:

C#

private async Task<List<ProductTypesData>> GetProductTypes()
{
   ...
}

[HttpGet]
public async Task<ActionResult<IEnumerable<ProductTypesData>>> GetAllProductTypes()
{
    return await this.GetProductTypes();
}

Or explicitly wrap the results in an ActionResult<T>:

C#

[HttpGet]
public async Task<ActionResult<IEnumerable<ProductTypesData>>> GetAllProductTypes()
{
    return Ok(await this.GetProductTypes());
}

Posted 6-Feb-23 22:19pm

Richard Deeming

Updated 6-Feb-23 23:39pm

v2

Comments

Agnib Pyne 2022 7-Feb-23 4:30am

While trying the above solution, am getting another error:

Awaited task returns 'interface System.Collections.Generic.IEnumerable<out t="">'
Exposes the enumerator, which supports a simple iteration over a collection of a specified type.

T is ProductTypesData

Richard Deeming 7-Feb-23 4:34am

That's not the complete error message.

Agnib Pyne 2022 7-Feb-23 4:56am

Awaited task returns 'interface System.Collections.Generic.IEnumerable<out t="">'
Exposes the enumerator, which supports a simple iteration over a collection of a specified type.

T is ProductTypesData
Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<api.classes.producttypesdata>' to 'Microsoft.AspNetCore.Mvc.ActionResult<system.collections.generic.ienumerable<api.classes.producttypesdata>>' [API]csharp(CS0029)

There are 2 error messages, one is the same as earlier one so I didn't post that.
But now I have mentioned the full error messages

Richard Deeming 7-Feb-23 5:03am

Which version of .NET are you using? According to the documentation, there is an implicit cast from T to ActionResult<T>, so there shouldn't be any problem implicitly converting IEnumerable<ProductTypesData> to ActionResult<IEnumerable<ProductTypesData>>. That should just work in everything from .NET Core 2.1 onwards.

Agnib Pyne 2022 7-Feb-23 5:25am

I am using .NET version 6.0.401 for this project.

Richard Deeming 7-Feb-23 5:37am

Aha! It looks like implicit conversions don't work for interfaces.

You'll either need to change the GetProductTypes return type, or explicitly wrap the result in an Ok(...) call.

Agnib Pyne 2022 7-Feb-23 6:01am

Yes, so can I change the return type of GetProductTypes to Task<actionresult<ienumerable<..>>>

Or how to wrap the result in Ok(...) call?

Richard Deeming 7-Feb-23 6:03am

No, change the return type to Task<List>ProductTypesData>>. You won't need to make any other changes to that method.

I've updated my solution with an example of that, and an example of wrapping the result in an Ok(...) call.

Agnib Pyne 2022 7-Feb-23 6:13am

Okay. Yes it worked now.
Thanks a lot.

Agnib Pyne 2022 7-Feb-23 4:57am

Yes. Can I mark the async method (GetProductTypes()) as Task<actionresult<ienumerable>> too? If not, then can I mark [HttpGet] method as Task<ienumerable<...>> only since am not using any HttpResponseMessage within the [HttpGet] method. I am just returning the data to the UI and dispaying it.

But, if I have to use HttpResponseMessage as well then ActinResult is needed. In that case, what needs to be done?