In the last post we found that the Startup class in ASP.NET Core works based on convention. The runtime inspects the class through reflection and invokes the appropriate methods i.e. Configure and ConfigureServices (optional). If class is not properly implemented e.g. say Configure method is not present then there should be a runtime exception. When I removed the Configure method from the Startup class I got the following error:
Unhandled Exception: System.InvalidOperationException: A public method named ‘ConfigureProduction’ or ‘Configure’ could not be found in the ‘startuptest.Startup’ type.
The above message is quite as expected but it was not clear to me why runtime was looking for a method named ConfigureProduction?
There seems to be something missing in the documentation and something else going on inside. So the best way to clear this confusion is to have a look into the ASP.NET Core codebase in github. The core logic for loading the Startup class is implemented in StartupLoader class as shown below.
Note, the last parameter environmentName. Does this ring a bell ? “ConfigureProduction”… Production…Environment.
Let’s drill down further. Let’s concentrate on the FindConfigureDelegate method which is used to identify the Configure method of the Startup class. This method in turn invokes “FindMethod” and we need to get into the logic of “FindMethod” to understand this better.
The code snippet of FindMethod is shown below.
The steps in selecting the right method are:
a) Look for method with name Configure{EnvironmentName}. If found, then check for overloads.In case there are overloads then throw InvalidOperationException.
b) If there are no method with name Configure{EnvironmentName} , then look for method with name Configure.If found, then check for overloads.In case there are overloads then throw InvalidOperationException.
c) If there are no methods found with name Configure{EnvironmentName} or Configure then throw InvalidOperationException.
Step c) above explains the exception message we started with “Unhandled Exception: System.InvalidOperationException: A public method named ‘ConfigureProduction’ or ‘Configure’ could not be found in the ‘startuptest.Startup’ type.”
The only missing piece in the puzzle is now the environment name. ASP.NET Core environment name is stored in the environment variable “ASPNETCORE_ENVIRONMENT”. The runtime reads from this variable and sets the IHostingEnvironment.EnvironmentName property. The framework supported values for this variable are “Development” , “Staging” and “Production” , however it can accept other values as well.
We can set this variable from the command line as
set ASPNETCORE_ENVIRONMENT=Development2
Now running the code without any Configure method in the Startup class will lead to the following exception message.
Unhandled Exception: System.InvalidOperationException: A public method named ‘ConfigureDevelopment2‘ or ‘Configure’ could not be found in the ‘startuptest.Startup’ type.
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