How it works
SelectMany can flatten nested collections, but it can also construct every combination of two independent sequences. For each color, the sample visits every size and projects a variant label. Two colors multiplied by three sizes produce six variants. This is a Cartesian product rather than a key-based join.
The output makes the nested-loop behavior visible. All three red sizes appear, followed by all three blue sizes. The result is useful for a small product-variant matrix, a test-case table, or another domain where every combination is actually required. The projection does not establish that those combinations are valid for a real inventory.
Multiplicative growth is the important cost boundary. A few small inputs are easy to inspect, but adding more independent dimensions can create a surprisingly large result. Materializing the whole product allocates storage for every combination. Calculate the expected size before turning a small demonstration into a production data-processing pipeline.
A common query bug accidentally omits a relationship condition and creates a product where a join was intended. The output may still look plausible because each row contains valid-looking values. Checking row counts and key relationships can reveal that error earlier than inspecting a handful of formatted rows.
Practice by adding a third color and then a fourth size. Predict the result count without executing the program. Next, introduce a rule that blue is not offered in size L and filter the combinations deliberately. Keep the compatibility rule visible so it is clear why the result is no longer the full product.
Finally, compare generating all combinations and filtering afterward with choosing a smaller compatible inner sequence for each outer item. Both can express the same final result, but their work and readability may differ. State the intended cardinality, test an empty dimension, and avoid assuming that a successful query implies an acceptably bounded workload.
A working example
using System;
using System.Linq;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Threading;
using System.Threading.Tasks;
using System.Runtime.CompilerServices;
string[] colors = { "red", "blue" };
string[] sizes = { "S", "M", "L" };
var variants = colors.SelectMany(color => sizes,
(color, size) => $"{color}-{size}").ToArray();
Console.WriteLine(variants.Length);
Console.WriteLine(string.Join(",", variants));
Expected output
6 red-S,red-M,red-L,blue-S,blue-M,blue-L
Each outer item visits the entire inner sequence.