I can use csc.exe to compile one cs file to a dll in .net framework...but how can i do this in dotnet core by dotnet cli?

At a minimum you'll need a .csproj file with this content:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>
</Project>

That is assuming you'll be targeting netstandard2.0 which would allow your library to run on .NET Framework 4.6.1, .NET Core 2.0 and any other runtime that implements the standard. And also have downloaded the .NET Core 2.0 SDK.

Unlike the old csproj, there's no need to add the individual .cs files to compile them.

So on a directory with such content:

• Class1.cs
• Class2.cs
• Project.csproj (with the content mentioned above)

Simply running: dotnet build will output a dll at the bin folder as you'd expect.

It's worth mentioning that the .NET Core CLI have templates for ASP.NET Core, class library, xUnit test project, etc:

$ dotnet new classlib

Would give you the csproj and a first class file.

You can use dotnet build to build your project for asp.net core