Resolving MSB3247 - Found conflicts between different versions of the same dependent assembly

Change the "MSBuild project build output verbosity" to "Detailed" or above. To do this, follow these steps:

1. Bring up the Options dialog (Tools -> Options...).
2. In the left-hand tree, select the Projects and Solutions node, and then select Build and Run.
   • Note: if this node doesn't show up, make sure that the checkbox at the bottom of the dialog Show all settings is checked.

3. In the tools/options page that appears, set the MSBuild project build output verbosity level to the appropriate setting depending on your version:

   • Diagnostics when on VS2012, VS2013 or VS2015 (the message in these versions says you should use "Detailed", but this is plain wrong, you should use "Diagnostics")
   • Detailed when you're on VS2010
   • Normal will suffice in VS2008 or older.
4. Build the project and look in the output window.

Check out the MSBuild messages. The ResolveAssemblyReferences task, which is the task from which MSB3247 originates, should help you debug this particular issue.

My specific case was an incorrect reference to SqlServerCe. See below. I had two projects referencing two different versions of SqlServerCe. I went to the project with the older version, removed the reference, then added the correct reference.

Target ResolveAssemblyReferences:
    Consider app.config remapping of assembly "System.Data.SqlServerCe, ..." 
        from Version "3.5.1.0" [H:\...\Debug\System.Data.SqlServerCe.dll] 
        to Version "9.0.242.0" [C:\Program Files\Microsoft Visual Studio 8\Common7\IDE\PublicAssemblies\System.Data.SqlServerCe.dll]
        to solve conflict and get rid of warning.
    C:\WINDOWS\Microsoft.NET\Framework\v3.5\Microsoft.Common.targets : 
        warning MSB3247: Found conflicts between different versions of the same dependent assembly.

You do not have to open each assembly to determine the versions of referenced assemblies.

• You can check the Properties of each Reference.
• Open the project properties and check the versions of the References section.
• Open the projects with a Text Editor.
• Use .Net Reflector.

Mike Hadlow has posted a little console app called AsmSpy that rather nicely lists each assembly's references:

Reference: System.Net.Http.Formatting
        4.0.0.0 by Shared.MessageStack
        4.0.0.0 by System.Web.Http

Reference: System.Net.Http
        2.0.0.0 by Shared.MessageStack
        2.0.0.0 by System.Net.Http.Formatting
        4.0.0.0 by System.Net.Http.WebRequest
        2.0.0.0 by System.Web.Http.Common
        2.0.0.0 by System.Web.Http
        2.0.0.0 by System.Web.Http.WebHost

This is a much quicker way to get to the bottom of the warning MSB3247, than to depend on the MSBuild output.

Sometimes AutoGenerateBindingRedirects isn't enough (even with GenerateBindingRedirectsOutputType). Searching for all the There was a conflict entries and fixing them manually one by one can be tedious, so I wrote a small piece of code that parses the log output and generates them for you (dumps to stdout):

// Paste all "there was a conflict" lines from the msbuild diagnostics log to the file below
const string conflictFile = @"C:\AssemblyConflicts.txt";

var sb = new StringBuilder();
var conflictLines = await File.ReadAllLinesAsync(conflictFile);
foreach (var line in conflictLines.Where(l => !String.IsNullOrWhiteSpace(l)))
{
    Console.WriteLine("Processing line: {0}", line);

    var lineComponents = line.Split('"');
    if (lineComponents.Length < 2) 
        throw new FormatException("Unexpected conflict line component count");

    var assemblySegment = lineComponents[1];
    Console.WriteLine("Processing assembly segment: {0}", assemblySegment);
    var assemblyComponents = assemblySegment
                              .Split(",")
                              .Select(kv => kv.Trim())
                              .Select(kv => kv.Split("=")
                              .Last())
                              .ToArray();

    if (assemblyComponents.Length != 4) 
        throw new FormatException("Unexpected conflict segment component count");

    var assembly = assemblyComponents[0];
    var version = assemblyComponents[1];
    var culture = assemblyComponents[2];
    var publicKeyToken = assemblyComponents[3];

    Console.WriteLine("Generating assebmly redirect for Assembly={0}, Version={1}, Culture={2}, PublicKeyToken={3}", assembly, version, culture, publicKeyToken);
    sb.AppendLine($"<dependentAssembly><assemblyIdentity name=\"{assembly}\" publicKeyToken=\"{publicKeyToken}\" culture=\"{culture}\" /><bindingRedirect oldVersion=\"0.0.0.0-{version}\" newVersion=\"{version}\" /></dependentAssembly>");
}

Console.WriteLine("Generated assembly redirects:");
Console.WriteLine(sb);

Tip: use MSBuild Binary and Structured Log Viewer and only generate binding redirects for the conflicts in the project that emits the warning (that is, only past those there was a conflict lines to the input text file for the code above [AssemblyConflicts.txt]).

Visual Studio for Mac Community addition:

As AMissico's answer requires changing the log level, and neither ASMSpy nor ASMSpyPlus are available as a cross-platform solution, here is a short addition for Visual Studio for Mac:

https://docs.microsoft.com/en-us/visualstudio/mac/compiling-and-building

It's in Visual Studio Community → Preferences... → Projects → Build Log → verbosity

If you have resharper, remove all unused reference on your solution.