Added new kb article resolving-microsoft-mshtml-assembly-error-telerik-report-designer (#1739)

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Co-authored-by: KB Bot <[email protected]>
Co-authored-by: IvetNikolova <[email protected]>
Co-authored-by: Todor Arabadzhiev <[email protected]>

Author: 4 people

knowledge-base/images/DeveloperCommandPrompt.png

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+---
+title: Fixing "Could not load file or assembly Microsoft.mshtml" Error
+description: Resolve the "Could not load file or assembly Microsoft.mshtml" error encountered in Telerik Report Designer when designing reports.
+type: how-to
+page_title: Resolving Microsoft.mshtml Assembly Error in Telerik Report Designer
+meta_title: Resolving Microsoft.mshtml Assembly Error in Telerik Report Designer
+slug: resolving-microsoft-mshtml-assembly-error-telerik-report-designer
+tags: telerik reporting, error handling, microsoft.mshtml, gac, assembly references
+res_type: kb
+ticketid: 1692750
+---
+
+## Environment
+
+<table>
+    <tbody>
+        <tr>
+            <td>Product</td>
+            <td>Progress® Telerik® Reporting</td>
+        </tr>
+    </tbody>
+</table>
+
+## Description
+
+I encounter the `Could not load file or assembly Microsoft.mshtml` error when working with Telerik Report Designer. This happens consistently while updating or designing reports, particularly when adding a subreport and using an [HTMLTextBox]({%slug telerikreporting/designing-reports/report-structure/htmltextbox/overview%}) for a different SQL query. The error forces the system to shut down, requiring a restart.
+
+## Solution
+
+To resolve the "Could not load file or assembly Microsoft.mshtml" error, follow these steps:
+
+### Step 1: Copy the Assembly to the Designer Folder
+
+1. Locate the `microsoft.mshtml.dll` assembly file on your system.
+1. Copy the file to the Telerik Report Designer folder.
+
+    ![Telerik Report Designer folder](images/TelerikReportDesignerFolder.png)
+
+1. Open the report file (for example, `SampleReport.trdp`) and select the HTMLTextBox to check if the error is resolved.
+
+### Step 2: Add Assembly Reference to the Configuration Files
+
+1. Open the `Telerik.ReportDesigner.exe.config` or `Telerik.ReportDesigner.x86.exe.config` file.
+1. Add the following assembly reference. Ensure the `<Telerik.Reporting>` section is properly registered:
+
+	````XML
+<configuration>
+		<configSections>
+			<section name="Telerik.Reporting" type="Telerik.Reporting.Configuration.ReportingConfigurationSection, Telerik.Reporting"
+					allowLocation="true" allowDefinition="Everywhere"/>
+		</configSections>
+		// ...
+		<Telerik.Reporting>
+			<AssemblyReferences>
+				<add name="Microsoft.mshtml" version="7.0.3300.0" />
+			</AssemblyReferences>
+		</Telerik.Reporting>
+		// ...
+````
+
+
+1. Save the changes and relaunch Telerik Report Designer.
+
+### Step 3: Install Microsoft.mshtml in the Global Assembly Cache (GAC)
+
+1. Open the Developer Command Prompt for Visual Studio.
+
+	![Developer Command Prompt](images/DeveloperCommandPrompt.png)
+
+1. Run the command `gacutil /l microsoft.mshtml` to check if the assembly exists in the GAC. It should display `Number of items = 0` if missing.
+1. Navigate to the directory containing the `microsoft.mshtml.dll` file.
+1. Execute the following command to install the assembly: `gacutil /i microsoft.mshtml.dll`.
+1. Verify installation with `gacutil /l microsoft.mshtml`.
+
+## See Also
+
+*[Global Assembly Cache (GAC)](https://learn.microsoft.com/en-us/dotnet/framework/app-domains/gac)
+*[Install an assembly into the global assembly cache](https://learn.microsoft.com/en-us/dotnet/framework/app-domains/install-assembly-into-gac)