Software⏱️ 3 min read📅 2026-06-04

How to Fix: DefaultInlineConstraintResolver Error in WebAPI 2

Resolve DefaultInlineConstraintResolver error in WebAPI 2 by configuring the IIS7.5 to use the correct pipeline and enabling the ASP.NET Web API module.

Quick Answer: Enable the ASP.NET Web API module in IIS7.5, configure the pipeline correctly, and ensure that the .NET Framework version matches the one used in Visual Studio.

📋 Table of Contents

  1. ⚠️ Common Causes
  2. 🔧 Proven Troubleshooting Steps
  3. 🎯 Final Words

The DefaultInlineConstraintResolver error in Web API 2 occurs when the inline constraint resolver is unable to resolve an inline constraint, such as the 'string' type, on a POST request to an API method using IIS 7.5.

This error affects developers who are using Web API 2 and IIS 7.5, causing issues with making GET requests to their APIs after successfully sending a POST request.

⚠️ Common Causes

  • The primary reason for this error is due to the way IIS 7.5 handles inline constraints in Web API 2. By default, IIS 7.5 does not support inline constraint resolution, which can cause issues with resolving constraints on GET requests.
  • Another possible cause is a misconfiguration of the Web API configuration or the IIS 7.5 settings.

🔧 Proven Troubleshooting Steps

Enabling ASP.NET Compatibility Mode

  1. Step 1: Open the IIS Manager and select the application pool that corresponds to your Web API project.
  2. Step 2: In the Application Pool Properties window, click on the 'Edit' button and then navigate to the 'Advanced settings' section.
  3. Step 3: Under the 'General' tab, set the '.NET CLR version' to '.NET 4.0 Integrated', and ensure that the 'Enable 32-bit App Domains' checkbox is unchecked.
  4. Step 4: Restart the application pool to apply the changes.

Configuring Web API to Use IIS Express

  1. Step 1: Open the Web.config file of your Web API project and add the following configuration element:
  2. Step 2: Open the IIS Manager, select the website that corresponds to your Web API project, and click on the 'Edit' button.
  3. Step 3: In the IIS Settings window, navigate to the 'Application Settings' section and add a new setting named 'aspNetCore' with a value of ''.
  4. Step 4: Restart the IIS service to apply the changes.

🎯 Final Words

To resolve the DefaultInlineConstraintResolver error in Web API 2, you can try enabling ASP.NET Compatibility Mode or configuring Web API to use IIS Express. By following these steps, you should be able to resolve the issue and make GET requests to your APIs successfully.

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