Determine .NET Framework version for dll

The simplest way: just open the .dll in any text editor. Look at one of the last lines:

If you have dotPeek from JetBrains, you can see it in Assembly Explorer.

I quickly wrote this C# console app to do this:

https://github.com/stuartjsmith/binarydetailer

Simply pass a directory as a parameter and it will do its best to tell you the net framework for each dll and exe in there

"Detect It Easy" also known as DiE is a program for determining types of files. Works with .dll files or other (.exe) files. Absolute free for commercial and non-commercial use.

Expanding on the answers here, this can blow up if there is a dependent assembly. If you're lucky and you know where the dependent is (or even luckier, it's in the GAC) then this may help ...

using System.Reflection;
using System.Runtime.Versioning;
// ...
{
    AppDomain.CurrentDomain.ReflectionOnlyAssemblyResolve += new ResolveEventHandler(CurrentDomain_ReflectionOnlyAssemblyResolve);
    var asm = System.Reflection.Assembly.LoadFrom(@"C:\Codez\My.dll");
    var targetFrameAttribute = asm.GetCustomAttributes(true).OfType<TargetFrameworkAttribute>().FirstOrDefault();
    targetFrameAttribute.Dump();
}

Assembly CurrentDomain_ReflectionOnlyAssemblyResolve(object sender, ResolveEventArgs args)
{
    var name = args.Name;

    if (name.StartsWith("Depends"))
        return System.Reflection.Assembly.ReflectionOnlyLoadFrom(@"C:\Codez\Depends.dll");

    return System.Reflection.Assembly.ReflectionOnlyLoad(args.Name);
}

Reference: https://weblog.west-wind.com/posts/2006/Dec/22/Reflection-on-Problem-Assemblies

Using the "read the text contents of the dll" approach:

private static readonly Regex CompiledNetCoreRegex = new Regex(@".NETCoreApp,Version=v[0-9\.]+", RegexOptions.Compiled);
private static readonly Regex CompiledNetFrameworkRegex = new Regex(@".NETFramework,Version=v[0-9\.]+", RegexOptions.Compiled);

// You can define other methods, fields, classes and namespaces here
public string GetTargetFramework(FileInfo dll)
{
    string contents = File.ReadAllText(dll.FullName);
 
    Match match = CompiledNetCoreRegex.Match(contents);
    if (match.Success)
    {
        return match.Value;
    }

    match = CompiledNetFrameworkRegex.Match(contents);
    if (match.Success)
    {
        return match.Value;
    }

    return "unable to compute target framework";
}