How to Fix the System.Web.Http.WebHost Assembly Loading Error in Azure

If you are developing an ASP.NET MVC web application and deploying it to Azure, you may encounter the following error when trying to access your website:

This error means that your web application cannot find the System.Web.Http.WebHost assembly, which is part of the ASP.NET Web API framework. This assembly is required for hosting Web API controllers and routing requests to them.

There are several possible causes and solutions for this error, depending on your project configuration and deployment settings. In this blog post, we will explain some of the common scenarios and how to fix them.

Scenario 1: You are using an outdated version of ASP.NET Web API

One of the possible causes of this error is that you are using an outdated version of ASP.NET Web API that does not match the version of System.Web.Http.WebHost assembly that is installed on your Azure web app.

ASP.NET Web API is a framework that allows you to create RESTful web services using .NET. It has been evolving over time and has different versions with different features and dependencies.

The System.Web.Http.WebHost assembly is part of the ASP.NET Web API framework and has different versions as well. The version of System.Web.Http.WebHost assembly that is installed on your Azure web app depends on the .NET Framework version that you have selected for your web app.

For example, if you have selected .NET Framework 4.7 for your web app, the version of System.Web.Http.WebHost assembly that is installed on your Azure web app is 5.2.7.0. If you have selected .NET Framework 4.5 for your web app, the version of System.Web.Http.WebHost assembly that is installed on your Azure web app is 4.0.0.0.

If your web application is using an older version of ASP.NET Web API that does not match the version of System.Web.Http.WebHost assembly that is installed on your Azure web app, you may encounter the assembly loading error.

To fix this error, you need to update your ASP.NET Web API framework to match the version of System.Web.Http.WebHost assembly that is installed on your Azure web app.

You can do this by using one of the following methods:

• Using NuGet Package Manager: This is the recommended method as it allows you to update the ASP.NET Web API framework and its dependencies with a few clicks. To use this method, follow these steps:
• Open your web application project in Visual Studio.
• Right-click on your project and select Manage NuGet Packages.
• In the Browse tab, search for Microsoft.AspNet.WebApi and select it.
• In the Version dropdown, select the latest stable version that matches the .NET Framework version that you have selected for your web app. For example, if you have selected .NET Framework 4.7 for your web app, select Microsoft.AspNet.WebApi 5.2.7.
• Click on Install and accept the license agreement.
• Repeat the same steps for Microsoft.AspNet.WebApi.Client and Microsoft.AspNet.WebApi.Core packages.
• Using Package Manager Console: This is an alternative method that allows you to update the ASP.NET Web API framework and its dependencies using PowerShell commands. To use this method, follow these steps:
• Open your web application project in Visual Studio.
• Go to Tools > NuGet Package Manager > Package Manager Console.
• In the Package Manager Console window, run the following command: Update-Package Microsoft.AspNet.WebApi -Version <version> Replace <version> with the latest stable version that matches the .NET Framework version that you have selected for your web app. For example, if you have selected .NET Framework 4.7 for your web app, use Microsoft.AspNet.WebApi 5.2.7.
• Repeat the same command for Microsoft.AspNet.WebApi.Client and Microsoft.AspNet.WebApi.Core packages.

After updating your ASP.NET Web API framework, rebuild your web application project and redeploy it to Azure. This should fix the assembly loading error.

Scenario 2: You are not copying the System.Web.Http.WebHost assembly to your output folder

Another possible cause of this error is that you are not copying the System.Web.Http.WebHost assembly to your output folder when building or publishing your web application project.

The System.Web.Http.WebHost assembly is a reference assembly that is required for hosting Web API controllers and routing requests to them. It is not part of the .NET Framework or Azure runtime, so it needs to be copied to your output folder along with your web application files.

If you are not copying the System.Web.Http.WebHost assembly to your output folder, your web application will not be able to find it and will throw the assembly loading error.

To fix this error, you need to ensure that the System.Web.Http.WebHost assembly is copied to your output folder when building or publishing your web application project.

You can do this by using one of the following methods:

• Using Visual Studio: This is the simplest method that allows you to copy the System.Web.Http.WebHost assembly to your output folder with a few clicks. To use this method, follow these steps:
• Open your web application project in Visual Studio.
• In the Solution Explorer, expand the References node and find the System.Web.Http.WebHost reference.
• Right-click on the System.Web.Http.WebHost reference and select Properties.
• In the Properties window, set the Copy Local property to True.
• Using MSBuild: This is an advanced method that allows you to copy the System.Web.Http.WebHost assembly to your output folder using MSBuild commands. To use this method, follow these steps:
• Open your web application project file (.csproj or .vbproj) in a text editor.
• Find the ItemGroup element that contains the System.Web.Http.WebHost reference. It should look something like this: <ItemGroup> <Reference Include="System.Web.Http.WebHost, Version=5.2.7.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL"> <HintPath>..\packages\Microsoft.AspNet.WebApi.WebHost.5.2.7\lib\net45\System.Web.Http.WebHost.dll</HintPath> </Reference> </ItemGroup>
• Add a Private element with a value of True inside the Reference element. It should look something like this: <ItemGroup> <Reference Include="System.Web.Http.WebHost, Version=5.2.7.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL"> <HintPath>..\packages\Microsoft.AspNet.WebApi.WebHost.5.2.7\lib\net45\System.Web.Http.WebHost.dll</HintPath> <Private>True</Private> </Reference> </ItemGroup>
• Save and close your project file.

After copying the System.Web.Http.WebHost assembly to your output folder, rebuild your web application project and redeploy it to Azure. This should fix the assembly loading error.

Conclusion

In this blog post, we have shown you how to fix the System.Web.Http.WebHost assembly loading error in Azure. We have explained some of the common scenarios and solutions for this error, such as updating your ASP.NET Web API framework, copying the System.Web.Http.WebHost assembly to your output folder, or using PowerShell compatibility mode. We hope this post has been helpful and informative for you. If you have any questions or feedback, please leave a comment below.