33 lines
1.5 KiB
C#
33 lines
1.5 KiB
C#
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namespace System.Web.Mvc.ExpressionUtil {
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using System;
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using System.Collections.Generic;
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using System.Linq.Expressions;
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// This is a visitor which rewrites constant expressions as parameter lookups. It's meant
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// to produce an expression which can be cached safely.
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internal sealed class HoistingExpressionVisitor<TIn, TOut> : ExpressionVisitor {
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private static readonly ParameterExpression _hoistedConstantsParamExpr = Expression.Parameter(typeof(List<object>), "hoistedConstants");
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private int _numConstantsProcessed;
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// factory will create instance
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private HoistingExpressionVisitor() { }
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public static Expression<Hoisted<TIn, TOut>> Hoist(Expression<Func<TIn, TOut>> expr) {
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// rewrite Expression<Func<TIn, TOut>> as Expression<Hoisted<TIn, TOut>>
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var visitor = new HoistingExpressionVisitor<TIn, TOut>();
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var rewrittenBodyExpr = visitor.Visit(expr.Body);
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var rewrittenLambdaExpr = Expression.Lambda<Hoisted<TIn, TOut>>(rewrittenBodyExpr, expr.Parameters[0], _hoistedConstantsParamExpr);
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return rewrittenLambdaExpr;
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}
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protected override Expression VisitConstant(ConstantExpression node) {
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// rewrite the constant expression as (TConst)hoistedConstants[i];
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return Expression.Convert(Expression.Property(_hoistedConstantsParamExpr, "Item", Expression.Constant(_numConstantsProcessed++)), node.Type);
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}
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}
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}
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