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