Populate IConfiguration for unit tests

You can use the following technique to mock IConfiguration.GetValue<T>(key) extension method.

var configuration = new Mock<IConfiguration>();
var configSection = new Mock<IConfigurationSection>();

configSection.Setup(x => x.Value).Returns("fake value");
configuration.Setup(x => x.GetSection("MySection")).Returns(configSection.Object);
//OR
configuration.Setup(x => x.GetSection("MySection:Value")).Returns(configSection.Object);

The solution I went for (which answers the question title at least!) is to use a settings file in the solution testsettings.json and set it to "Copy Always".

private IConfiguration _config;

public UnitTestManager()
{
    IServiceCollection services = new ServiceCollection();

    services.AddSingleton<IConfiguration>(Configuration);
}

public IConfiguration Configuration
{
    get
    {
        if (_config == null)
        {
            var builder = new ConfigurationBuilder().AddJsonFile($"testsettings.json", optional: false);
            _config = builder.Build();
        }

        return _config;
    }
}

You can use MemoryConfigurationBuilderExtensions to provide it via a dictionary.

using Microsoft.Extensions.Configuration;

var myConfiguration = new Dictionary<string, string>
{
    {"Key1", "Value1"},
    {"Nested:Key1", "NestedValue1"},
    {"Nested:Key2", "NestedValue2"}
};

var configuration = new ConfigurationBuilder()
    .AddInMemoryCollection(myConfiguration)
    .Build();

The equivalent JSON would be:

{
  "Key1": "Value1",
  "Nested": {
    "Key1": "NestedValue1",
    "Key2": "NestedValue2"
  }
}

The equivalent Environment Variables would be (assuming no prefix / case insensitive):

Key1=Value1
Nested__Key1=NestedValue1
Nested__Key2=NestedValue2

The equivalent Command Line parameters would be:

dotnet myapp.dll \
  -- \
  --Key1=Value1 \
  --Nested:Key1=NestedValue1 \
  --Nested:Key2=NestedValue2