方法 : 式ツリーを使用して動的クエリをビルドする

[アーティクル]
08/16/2008

更新 : 2007 年 11 月

このトピックでは、式ツリーを使用して動的 LINQ クエリを作成する方法について説明します。動的クエリは、コンパイル時にクエリの詳細がわからない場合に便利です。たとえば、あるアプリケーションに、エンドユーザーがデータをフィルタ処理するために 1 つ以上の述語を指定できるようなユーザーインターフェイスが用意されているとします。LINQ を使用してクエリを行うために、この種のアプリケーションでは、実行時に式ツリーを使用して LINQ クエリを作成する必要があります。

使用例

式ツリーを使用して IQueryable データソースに対するクエリを作成し、その後それを実行する方法を次の例に示します。コードは、次のクエリを表す式ツリーをビルドします。

C# クエリ

companies.Where(company => (company.ToLower() == "coho winery" || company.Length > 16)).OrderBy(company => company)

Visual Basic クエリ

companies.Where(Function(company) company.ToLower() = "coho winery" OrElse company.Length > 16).OrderBy(Function(company) company)

クエリ全体を構成する式を表す式ツリーを作成するために、System.Linq.Expressions 名前空間のファクトリメソッドが使用されます。標準クエリ演算子のメソッドの呼び出しを表す式は、そのメソッドの Queryable 実装を参照します。最終的な式ツリーが IQueryable データソースのプロバイダの CreateQuery<TElement>(Expression) 実装に渡され、IQueryable 型の実行可能クエリが作成されます。結果を取得するには、そのクエリ変数を列挙します。

Dim companies() As String = _
    {"Consolidated Messenger", "Alpine Ski House", "Southridge Video", "City Power & Light", _
                       "Coho Winery", "Wide World Importers", "Graphic Design Institute", "Adventure Works", _
                       "Humongous Insurance", "Woodgrove Bank", "Margie's Travel", "Northwind Traders", _
                       "Blue Yonder Airlines", "Trey Research", "The Phone Company", _
                       "Wingtip Toys", "Lucerne Publishing", "Fourth Coffee"}

' The IQueryable data to query.
Dim queryableData As IQueryable(Of String) = companies.AsQueryable()

' Compose the expression tree that represents the parameter to the predicate.
Dim pe As ParameterExpression = Expression.Parameter(GetType(String), "company")

' ***** Where(Function(company) company.ToLower() = "coho winery" OrElse company.Length > 16) *****
' Create an expression tree that represents the expression: company.ToLower() = "coho winery".
Dim left As Expression = Expression.Call(pe, GetType(String).GetMethod("ToLower", System.Type.EmptyTypes))
Dim right As Expression = Expression.Constant("coho winery")
Dim e1 As Expression = Expression.Equal(left, right)

' Create an expression tree that represents the expression: company.Length > 16.
left = Expression.Property(pe, GetType(String).GetProperty("Length"))
right = Expression.Constant(16, GetType(Integer))
Dim e2 As Expression = Expression.GreaterThan(left, right)

' Combine the expressions to create an expression tree that represents the
' expression: company.ToLower() = "coho winery" OrElse company.Length > 16).
Dim predicateBody As Expression = Expression.OrElse(e1, e2)

' Create an expression tree that represents the expression:
' queryableData.Where(Function(company) company.ToLower() = "coho winery" OrElse company.Length > 16)
Dim whereCallExpression As MethodCallExpression = Expression.Call( _
        GetType(Queryable), _
        "Where", _
        New Type() {queryableData.ElementType}, _
        queryableData.Expression, _
        Expression.Lambda(Of Func(Of String, Boolean))(predicateBody, New ParameterExpression() {pe}))
' ***** End Where *****

' ***** OrderBy(Function(company) company) *****
' Create an expression tree that represents the expression:
' whereCallExpression.OrderBy(Function(company) company)
Dim orderByCallExpression As MethodCallExpression = Expression.Call( _
        GetType(Queryable), _
        "OrderBy", _
        New Type() {queryableData.ElementType, queryableData.ElementType}, _
        whereCallExpression, _
        Expression.Lambda(Of Func(Of String, String))(pe, New ParameterExpression() {pe}))
' ***** End OrderBy *****

' Create an executable query from the expression tree.
Dim results As IQueryable(Of String) = queryableData.Provider.CreateQuery(Of String)(orderByCallExpression)

' Enumerate the results.
For Each company As String In results
    Console.WriteLine(company)
Next

' This code produces the following output:
'
' Blue Yonder Airlines
' City Power & Light
' Coho Winery
' Consolidated Messenger
' Graphic Design Institute
' Humongous Insurance
' Lucerne Publishing
' Northwind Traders
' The Phone Company
' Wide World Importers

string[] companies = { "Consolidated Messenger", "Alpine Ski House", "Southridge Video", "City Power & Light",
                   "Coho Winery", "Wide World Importers", "Graphic Design Institute", "Adventure Works",
                   "Humongous Insurance", "Woodgrove Bank", "Margie's Travel", "Northwind Traders",
                   "Blue Yonder Airlines", "Trey Research", "The Phone Company",
                   "Wingtip Toys", "Lucerne Publishing", "Fourth Coffee" };

// The IQueryable data to query.
IQueryable<String> queryableData = companies.AsQueryable<string>();

// Compose the expression tree that represents the parameter to the predicate.
ParameterExpression pe = Expression.Parameter(typeof(string), "company");

// ***** Where(company => (company.ToLower() == "coho winery" || company.Length > 16)) *****
// Create an expression tree that represents the expression 'company.ToLower() == "coho winery"'.
Expression left = Expression.Call(pe, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
Expression right = Expression.Constant("coho winery");
Expression e1 = Expression.Equal(left, right);

// Create an expression tree that represents the expression 'company.Length > 16'.
left = Expression.Property(pe, typeof(string).GetProperty("Length"));
right = Expression.Constant(16, typeof(int));
Expression e2 = Expression.GreaterThan(left, right);

// Combine the expression trees to create an expression tree that represents the
// expression '(company.ToLower() == "coho winery" || company.Length > 16)'.
Expression predicateBody = Expression.OrElse(e1, e2);

// Create an expression tree that represents the expression
// 'queryableData.Where(company => (company.ToLower() == "coho winery" || company.Length > 16))'
MethodCallExpression whereCallExpression = Expression.Call(
    typeof(Queryable),
    "Where",
    new Type[] { queryableData.ElementType },
    queryableData.Expression,
    Expression.Lambda<Func<string, bool>>(predicateBody, new ParameterExpression[] { pe }));
// ***** End Where *****

// ***** OrderBy(company => company) *****
// Create an expression tree that represents the expression
// 'whereCallExpression.OrderBy(company => company)'
MethodCallExpression orderByCallExpression = Expression.Call(
    typeof(Queryable),
    "OrderBy",
    new Type[] { queryableData.ElementType, queryableData.ElementType },
    whereCallExpression,
    Expression.Lambda<Func<string, string>>(pe, new ParameterExpression[] { pe }));
// ***** End OrderBy *****

// Create an executable query from the expression tree.
IQueryable<string> results = queryableData.Provider.CreateQuery<string>(orderByCallExpression);

// Enumerate the results.
foreach (string company in results)
    Console.WriteLine(company);

/*  This code produces the following output:

    Blue Yonder Airlines
    City Power & Light
    Coho Winery
    Consolidated Messenger
    Graphic Design Institute
    Humongous Insurance
    Lucerne Publishing
    Northwind Traders
    The Phone Company
    Wide World Importers
*/

このコードでは、Queryable.Where メソッドに渡す述語に一定数の式を使用します。しかし、ユーザー入力によって数が増減する述語式を結合するアプリケーションを作成することもできます。また、ユーザー入力に応じて、クエリで呼び出される標準クエリ演算子を変更することもできます。