I'm trying to get a query string value using:
_httpContextAccessor.HttpContext.Request.QueryString["data"]
but it fails with error:
Cannot apply indexing with [] to an expression of type 'QueryString'
QueryString is from the Microsoft.AspNetCore.Http namespace.
Generally, you should rely on model-binding to access incoming values, not read them explicitly from a certain request source.
However, the correct way to read query-string values is through Request.Query instead. And in your case:
_httpContextAccessor.HttpContext.Request.Query["data"]
See Model-Binding