System.IO.FileNotFoundException – Could not load file or assembly ‘xyz’

The System.IO.FileNotFoundException – Could not load file or assembly 'xyz' is a runtime exception in C# that occurs when the application tries to load a file or assembly that cannot be found. This typically happens when:

1. The file or assembly is missing from the expected location.
2. The file or assembly is not included in the build output.
3. The file or assembly is not properly referenced in the project.

Here’s how you can troubleshoot and fix this issue:

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1. Check File or Assembly Path

• Ensure that the file or assembly exists in the expected location. Example:

   var assembly = System.Reflection.Assembly.LoadFrom("MissingAssembly.dll"); // Error: File not found

Fix:

   var assembly = System.Reflection.Assembly.LoadFrom("ExistingAssembly.dll"); // Ensure file exists

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2. Include Files in Build Output

• Ensure that the file or assembly is included in the build output (e.g., set the file’s Copy to Output Directory property to Copy always or Copy if newer in Visual Studio). Example:
• In Visual Studio:
  1. Right-click the file in Solution Explorer.
  2. Select Properties.
  3. Set Copy to Output Directory to Copy always or Copy if newer.

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3. Check Project References

• Ensure that the assembly is properly referenced in the project. Example:
• In Visual Studio:
  1. Right-click the project in Solution Explorer.
  2. Select Add > Reference.
  3. Add the missing assembly.

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4. Check for Missing Dependencies

• Ensure that all dependencies of the assembly are available. Use tools like Fusion Log Viewer to diagnose missing dependencies. Example:
• Use Fusion Log Viewer to identify missing dependencies:
  1. Open Developer Command Prompt.
  2. Run fuslogvw to open Fusion Log Viewer.
  3. Check the logs for missing assemblies.

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5. Use AppDomain.CurrentDomain.AssemblyResolve

• Handle the AssemblyResolve event to dynamically load missing assemblies. Example:

   AppDomain.CurrentDomain.AssemblyResolve += (sender, args) =>
   {
       string assemblyPath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "MissingAssembly.dll");
       if (File.Exists(assemblyPath))
       {
           return Assembly.LoadFrom(assemblyPath);
       }
       return null;
   };

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Example of Correct Code

using System;
using System.IO;
using System.Reflection;

public class Program
{
    public static void Main(string[] args)
    {
        // Example 1: Check file or assembly path
        string assemblyPath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "ExistingAssembly.dll");
        if (File.Exists(assemblyPath))
        {
            var assembly = Assembly.LoadFrom(assemblyPath);
            Console.WriteLine("Assembly loaded");
        }
        else
        {
            Console.WriteLine("Assembly not found");
        }

        // Example 2: Handle missing assemblies dynamically
        AppDomain.CurrentDomain.AssemblyResolve += (sender, args) =>
        {
            string missingAssemblyPath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "MissingAssembly.dll");
            if (File.Exists(missingAssemblyPath))
            {
                return Assembly.LoadFrom(missingAssemblyPath);
            }
            return null;
        };

        try
        {
            var missingAssembly = Assembly.Load("MissingAssembly");
            Console.WriteLine("Missing assembly loaded dynamically");
        }
        catch (FileNotFoundException ex)
        {
            Console.WriteLine("Error: " + ex.Message);
        }
    }
}

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