How to Perform a Full Outer Join on Two Lists Using LINQ?

linq -all external connection

Question:

How to use Linq to perform a full external connection on the two object lists, even if some objects in the list exist?

Explanation:

Internal connection: Elements with corresponding keys in the two lists are eliminated to exclude elements without corresponding elements.

External connection (left connection):
• Including the elements in the left list, there is no corresponding element even in the right list. All external connection:
Including the elements in two lists, whether they have corresponding elements in another list.
• Implement:
• The following code provides a general extension method to perform all external connection:

This method uses the following parameters:

Two lists A and B to be connected.

The key Selectkeya and Selectkeyb, specify which attribute application of the objects in A and B as the connection key.

<code class="language-csharp">internal static IEnumerable<TResult> FullOuterJoin<TA, TB, TKey, TResult>(
    this IEnumerable<TA> a,
    IEnumerable<TB> b,
    Func<TA, TKey> selectKeyA,
    Func<TB, TKey> selectKeyB,
    Func<TA, TB, TKey, TResult> projection,
    TA defaultA = default(TA),
    TB defaultB = default(TB),
    IEqualityComparer<TKey> cmp = null)
{
    cmp = cmp ?? EqualityComparer<TKey>.Default;
    var alookup = a.ToLookup(selectKeyA, cmp);
    var blookup = b.ToLookup(selectKeyB, cmp);

    var keys = new HashSet<TKey>(alookup.Select(p => p.Key), cmp);
    keys.UnionWith(blookup.Select(p => p.Key));

    var join = from key in keys
               from xa in alookup[key].DefaultIfEmpty(defaultA)
               from xb in blookup[key].DefaultIfEmpty(defaultB)
               select projection(xa, xb, key);

    return join;
}</code>

Project function project, specify how to convert the connected element into the result type Tresult.

The optional default value DefaultA and DefaultB, if there is no corresponding element in another list, use these values.

• Optional key comparison CMP, which is used to specify the custom key comparison comparison.
Usage:
• To use this expansion method, just call it on the first list A:
This will generate the required output:

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