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Union type

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Union type

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Some API parameters within Elasticsearch can accept more than one JSON data structure, for example, source filtering on a search request can accept

a bool value to disable _source retrieval
a string value representing a wildcard pattern to control what parts of _source to return
an array of string values representing multiple wildcard patterns to control what parts of _source to return
an object with includes and excludes properties that each accept an array of wildcard patterns to control what parts of _source to include and exclude, respectively.

That’s a lot of different flexibility! NEST includes a Union<TFirst,TSecond> type to make it easier to work with these kinds of parameters, forming the union of two types, TFirst and TSecond.

Implicit conversion

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The Union<TFirst,TSecond> has implicit operators to convert from an instance of TFirst or TSecond to an instance of Union<TFirst,TSecond>. This is often the easiest way of construction an instance

Union<bool, ISourceFilter> sourceFilterFalse = false;

Union<bool, ISourceFilter> sourceFilterInterface = new SourceFilter
{
    Includes = new [] { "foo.*" }
};

Constructor

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Sometimes, the constructor of Union<TFirst,TSecond> may be required in cases where the compiler is unable to infer the correct implicit conversion

var sourceFilterTrue = new Union<bool, ISourceFilter>(true);

var sourceFilterInterface = new Union<bool, ISourceFilter>(new SourceFilter
{
    Includes = new [] { "foo.*" }
});

Match

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The Match method can be used to operate on the value encapsulated by the instance of Union<TFirst,TSecond>. Two delegates are passed; one to operate on a TFirst value and the other to operate on a TSecond value.

sourceFilterTrue.Match(
    b => b.Should().BeTrue(),
    s => s.Should().BeNull());

sourceFilterInterface.Match(
    b => b.Should().BeFalse(),
    s => s.Should().NotBeNull());

The delegates can also return a value

var serializedFilterTrue = sourceFilterTrue.Match(
    b => serializer.SerializeToString(b),
    s => null);

serializedFilterTrue.Should().Be("true");

var serializedFilterInterface = sourceFilterTrue.Match(
    b => null,
    s => serializer.SerializeToString(s));

serializedFilterInterface.Should().Be("{\"includes\":[\"foo.*\"]}");

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