Cannot implicitly convert type 'Microsoft.AspNetCore.Mvc.BadRequestObjectResult'

Actually you need to return IActionResult instead of IList for below ASP.NET Core 2.1,

public async Task<IActionResult> Create([FromBody]TodoCreateViewModel model)

Then it will work.

And for ASP.NET Core 2.1 as suggested by @amankkg,

public async Task<ActionResult<IList<Todo>>> Create([FromBody]TodoCreateViewModel model)

For ASP.NET Core 2.1, you should use ActionResult<T> but there is a limitation with Interface's.

This Works

public ActionResult<IList<string>> Create()
{  
    return new List<string> { "value1", "value2" };
}

Doesn't Work

public ActionResult<IList<string>> Create()
{
    //DOESN'T COMPILE:
    //Error CS0029  Cannot implicitly convert type
    //'System.Collections.Generic.IList<string>' 
    //to 'Microsoft.AspNetCore.Mvc.ActionResult<System.Collections.Generic.IList<string>>'
    //the cast here is for demo purposes.
    //the problem will usually arise from a dependency that returns
    //an interface.
    var result = new List<string> { "value1", "value2" }
                     as IList<string>;
    return result;
}

C# doesn't support implicit cast operators on interfaces. Consequently, conversion of the interface to a concrete type is necessary to use ActionResult.

Source: ActionResult type

---

• Demo project
• Interesting read on C# doesn't support implicit cast operators on interfaces

---

Sidenote: you don't need [FromBody] as ASP.NET will do that automatically. More here.

Your action return type does not take in mind possible BadRequest.

Instead of direct usage of IList<Todo> you need to wrap it with generic ActionResult type.

public async Task<ActionResult<IList<Todo>>> Create(...

Here are the related docs.