MigrateToSdkProject.cs

void Main()
{
	var files = Directory.EnumerateFiles(@"C:\Users\me\Documents\Repos\myrepo\", "*.csproj", SearchOption.AllDirectories);
	foreach (var csprojPath in files)
	{
		PatchCsProj(csprojPath);
	}
}

void PatchCsProj(string path) 
{
	var directory = Path.GetDirectoryName(path);
	var oldCsProj = File.ReadAllText(path);
	if (oldCsProj.Contains("<TargetFramework>net45</TargetFramework>"))
	{
		return;
	}
	var packagesPath = Path.Combine(directory, "packages.config");
	var packagesconfig = File.Exists(packagesPath) ? File.ReadAllText(packagesPath) : string.Empty;
	var packages = Regex.Matches(packagesconfig, "id=\"([^\"]+)\"\\s+version=\"([^\"]+)\"").Cast<Match>().Select(m => new {Name = m.Groups[1].Value, Version = m.Groups[2].Value})
						.Where(m => !m.Name.StartsWith("System")).ToArray();

	var projectReferences = Regex.Matches(oldCsProj, "ProjectReference Include=\"([^\"]+)\"").Cast<Match>().Select(m => m.Groups[1].Value).ToArray();

	//(packages, projectReferences).Dump();
	
	const string csprojTemplate = @"<Project Sdk=""Microsoft.NET.Sdk"">
  <PropertyGroup>
    <TargetFramework>net45</TargetFramework>
    <LangVersion>8.0</LangVersion>
  </PropertyGroup>
  
  <ItemGroup>
{0}
  </ItemGroup>
  
  <ItemGroup>
{1}
  </ItemGroup>
</Project>";

	var sdkPackages = string.Join(Environment.NewLine, packages.Select(p => $"    <PackageReference Include=\"{p.Name}\" Version=\"{p.Version}\" />"));
	var sdkProjectReferences = string.Join(Environment.NewLine, projectReferences.Select(pr => $"    <ProjectReference Include=\"{pr}\" />"));
	
	var resultingCsProj = string.Format(csprojTemplate, sdkPackages, sdkProjectReferences);
	File.WriteAllText(path, resultingCsProj);
	File.Delete(packagesPath);
	Directory.Delete(Path.Combine(directory, "Properties"), true);
}