> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # C# From Clause The `from` clause is the mandatory starting point of any Language Integrated Query (LINQ) expression in C#. It establishes the data source for the query and introduces a strongly typed range variable that represents each successive element within that source sequence during iteration. ```csharp theme={"dark"} from [type] rangeVariable in dataSource ``` ## Technical Mechanics **1. Data Source Constraints** C# LINQ relies on the **Query Expression Pattern** (duck typing). The `dataSource` specified in the `from` clause is not strictly required to implement `IEnumerable`, `IEnumerable`, or `IQueryable`. Instead, the type must expose methods with specific signatures (such as `Select`, `SelectMany`, or `Where`) that match the operations performed in the query expression. **2. The Range Variable** The `rangeVariable` functions similarly to the iteration variable in a `foreach` statement. Like a `foreach` iteration variable, the range variable in a LINQ query is strictly read-only (immutable); it cannot be reassigned or modified within the query expression. **3. Type Inference and Explicit Typing** In most queries, the C# compiler implicitly infers the type of the range variable based on the generic type argument of the data source. ```csharp theme={"dark"} IEnumerable words = new[] { "alpha", "beta" }; // The compiler infers 'w' is of type string var query = from w in words select w; ``` Explicit typing is syntactically permitted and becomes strictly necessary when querying legacy, non-generic collections (such as `System.Collections.ArrayList`). Explicitly typing the range variable instructs the compiler to inject a call to the `.Cast()` method, which performs an **explicit cast** `(T)element` internally during iteration. ```csharp theme={"dark"} System.Collections.ArrayList mixedTypes = new System.Collections.ArrayList { "alpha", "beta" }; // Explicitly typing 'w' as string translates to mixedTypes.Cast() var query = from string w in mixedTypes select w; ``` ## Compound `from` Clauses A single LINQ query expression can contain multiple `from` clauses. Subsequent `from` clauses are used to query nested collections or to perform cross-joins. When a second `from` clause is introduced, it generates a Cartesian product of the first data source and the second data source. At compile time, the C# compiler translates compound `from` clauses into calls to the `SelectMany` standard query operator. ```csharp theme={"dark"} class Container { public List Items { get; set; } = new List(); } IEnumerable containers = new List { new Container { Items = new List { 1, 2 } }, new Container { Items = new List { 3, 4 } } }; // Compound 'from' clauses flatten the nested sequences var flattenedQuery = from c in containers from item in c.Items select item; ``` In the example above, the first `from` clause introduces the range variable `c` (representing a `Container`). The second `from` clause reaches into `c` to iterate over its `Items` property, introducing a new range variable `item`. Because query expression translation is purely syntactic and occurs before type binding, the C# compiler translates this syntax to include the result selector: `containers.SelectMany(c => c.Items, (c, item) => item)`. ## Scope and Visibility The scope of the range variable begins at the `from` clause that declares it and extends to the end of the query expression (typically terminated by a `select` or `group` clause). If query continuation is used (via the `into` keyword), the original range variable falls out of scope, and a new range variable must be defined for the continuation.

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