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remi bourgarel
remi bourgarel

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Blazor How It Works Part 2

How does Blazor works ? Part 2 : building

This blog post is based on net core sdk 3.0.100-preview5-011568, some stuff might change in the future.

This blog post is the 2nd part of my serie about how blazor (client side) works, from the build process to the update of the UI. The first part started with a “simple” command “dotnet run” and ended with generated classes representing the razor files. Now we’ll have a look at the rest of the building process and the part of it that handle integration with monowasm.

For writing the first blog post I started from the output of the “dotnet build” command and tried to look for things related to Components and Blazor. I stopped my blog post at the step called “RazorGenerateComponentDeclaration”. After this step we can find many steps related to Blazor :

  • RazorCompileComponentDeclaration
  • _ResolveComponentRazorGenerateInputs
  • _CheckForIncorrectComponentsConfiguration
  • _PrepareBlazorOutputConfiguration
  • _DefineBlazorCommonInputs
  • _GenerateLinkerDescriptor
  • _CollectBlazorLinkerDescriptors
  • _LinkBlazorApplication
  • _BlazorResolveOutputBinaries
  • _ResolveBlazorBootJsonInputs
  • _GenerateBlazorBootJson
  • _BlazorCopyFilesToOutputDirectory
  • _BlazorBuildReport


This is declared in “Sdks\Microsoft.NET.Sdk.Razor\build\netstandard2.0\Microsoft.NET.Sdk.Razor.Component.targets” line 189. This task is basically a roslyn (c# compiler) task. That builds the components generated before and the other c# classes into an assembly. The list of the file is located in the variable “_RazorComponentDeclaration”. The result assembly will be located in the obj folder (variable BaseIntermediateOutputPath on file Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.DefaultOutputPaths.targets line 37). I won’t go into the roslyn compiler details here, it’s way above my skills.


This task setup 2 MSBuild element :

  • ItemGroup RazorGenerateWithTargetPath is setup for containing the path where the component classes were generated
  • PropertyGroup _RazorComponentDeclarationAssemblyFullPath contains the path of the assembly generated on the previous step.


This task does only one thing : display a warning (RAZORSDK1005 : “One or more Razor component files (.razor) were found, but the project is not configured to compile Razor Components. Configure the project by targeting RazorLangVersion 3.0 or newer. For more information, see”) if there is any razor file on the project and the project is not configured for building razor files.

This can happen if you remove the tag “RazorLangVersion” on your project file.


This task is not defined in the Sdk, if you look for the task name on your Sdk file, you won’t find it, but where is it ? If you look at the build log you can see there is the task location in the logs (my windows is located in French):

” dans le fichier “C:\Users\remi.nuget\packages\\3.0.0-preview5-19227-01\targets\Blazor.MonoRuntime.targets” du projet “C:\Users\remi\Source\Repos\Toss\Toss.Client\Toss.Client.csproj” “

So, this task is defined in a nuget package, but how does it works ?

  • First you need to add a reference to Blazor.Build nuget package for every Blazor project
  • The Blazor.Build nuget package has a “build” folder that is, by convention, read by MSBuild and it contains .targets file
  • This file content is simple and include the file All.targets located in the folder called “targets” on the nuget package.
  • This file and the other it includes (Blazor.MonoRuntime.targets and Publish.targets) define all the other required for Blazor to run
  • The task _PrepareBlazorOutputConfiguration has the attribute “AfterTargets” which hook a task on an other tasks. It’s one of the extension mechanism of MSBuild.

This task is well commented and prepares all the paths that will be needed for the “final” output of the build (Blazor.MonoRuntime.targets line 119). Basically it creates many path (like the blazor.boot.json path) and creates the “intermediate” output path “/obj/configuration/targetframework/blazor/”.

We can see on this task that mono.wasm and mono.js (clr implementation on WebAssembly) are taken from the nuget package Microsoft.AspNetCore.Blazor.Mono. This is defined by the variable MonoWasmRuntimePath in\Microsoft.AspNetCore.Blazor.Mono.props on the package Microsoft.AspNetCore.Blazor.Mono.

We can now understand how the aspnet core team updates the monowasm runtime for Blazor :

  • They build Microsoft.AspNetCore.Blazor.Mono with the latest monowasm version
  • They ship a new package including mono.wasm
  • They update Blazor.Build package to the newest Blazor.Mono version
  • They ship a new version of Blazor.Build


This tasks writes 3 files :

  • One with a hash of all the dependencies, the current assembly, the linking and debugging setting (\obj\Debug\netstandard2.0\blazor\inputs.basic.cache)
  • One with all the dependencies path (\obj\Debug\netstandard2.0\blazor\inputs.copylocal.txt)
  • And one with the linking option for the current build (\obj\Debug\netstandard2.0\blazor\inputs.linkerswitch.cache)


This task create a XML configuration file for the “linker” (we’ll see soon what it is) : \obj\Debug\netstandard2.0\blazor\linker.descriptor.xml.

This file looks like this

   <assembly fullname="Toss.Client" />

It is just the full name of the generated assembly.


This task creates a file (\obj\Debug\netstandard2.0\blazor\inputs.linker.cache) with a hash of all the dependant assemblies and the linker settings. At some point I don’t understand why they need all these files with hash, it seems that the hash in inputs.basic.cache is nearly about the same thing. But I must be wrong :)


This is where the “linker” is called. First it gather the project assembly file name, its dependencies and the BCL (base class library, it contains the basic class for the runtime such as DateTime, String or File). Then it calls the linker with a command like this :

dotnet ${Blazor.Mono path}/tools/illink/illink.dll {path to output directory blazor/linker} {path to all the assemblies} {path to previously created XML with assemblyname} {additionnal options}

The linker source code is defined here ( The readme says all about it “The IL Linker is a tool one can use to only ship the minimal possible IL code and metadata that a set of programs might require to run as opposed to the full libraries.”. As per the documentation the linking process is the following :

  • It loads all the types, fields, methods from the given context (assembly)
  • For all of them and the other elements they are using (recursively) it will decide how to annotate it : linked or not, to be processed or not.
  • Then depending on the previous decision some element (types, method …) will be removed
  • Then new assemblies will be created based on this decisionI think this is a bit like tree-shaking in angular/node projects.


This task consist of 2 other tasks : _CollectLinkerOutputs or _CollectResolvedAssemblies

_CollectLinkerOutputs assembly list from the linker output, and create their target path in “bin\Debug\netstandard2.0\dist_framework_bin”.

_CollectResolvedAssemblies is the same as _CollectLinkerOutputs but runs only if the linker is disabled and reads the assembly list from the depencency list.


This task writes a bunch of settings (configuration, assembly and pdb paths, linker enabed, debug enabled) to a text file in “obj\Debug\netstandard2.0\blazor\input.bootjson.cache”.


This task writes (again) a bunch of file with assembly and pdb path. I think I can find at least 10 file with dll path in this build process :). Then it calls this Program with the first parameter “write-boot-json”. This parameter causes the execution of this command which reads the file input (from previous steo) and creates the json file in “obj\Debug\netstandard2.0\blazor”


This step is pretty straightforward : during the whole build process a variable called BlazorItemOuput was filled with a list of source files and their destinations. This task reads this variable and then copies the source file to its destination, mostly from obj to bin. This is used for keeping only the file usefull at runtime from the obj folder.


This task displays the number of file copied previously and list all the said files.


This article was fun to write because it helps understand what is going on in Microsoft people head. It’s also nice for understanding a bit more about the building process in all the .net project.

It can also be helpful for debugging stuff when you have an error with sdk / target files or something. And maybe it’ll be a starting point for people looking for extension point in the blazor building process (changing dll extension ?).

I understand I might be vague on some steps, if you need more details please ask below I’ll be happy to dig a bit more.

Next blog posts will be about runtime.

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