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2013-03-04T11:05:46.623
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<p>No need to use Expression.Call, you can directly construct Expression Tree instead; I have create a static method which help me generate dynamic query:</p> <pre><code>public static void Test(string[] args) { using (var db = new DBContext()) { //query 1 var query1 = db.PrizeTypes.Where(m => m.rewards == 1000).Select(t => t.name); //query 2 which equal to query 1 Expression<Func<PrizeType, bool>> predicate1 = m => m.rewards == 1000; Expression<Func<PrizeType, string>> selector1 = t => t.name; var query2 = db.PrizeTypes.Where(predicate1).Select(selector1); Console.WriteLine(predicate1); Console.WriteLine(selector1); Console.WriteLine(); //query 3 which equal to query 1 and 2 Expression<Func<PrizeType, bool>> predicate2 = GetPredicateEqual<PrizeType>("rewards", (Int16)1000); Expression<Func<PrizeType, string>> selector2 = GetSelector<PrizeType, string>("name"); var query3 = db.PrizeTypes.Where(predicate2).Select(selector2); Console.WriteLine(predicate2); Console.WriteLine(selector2); //as you can see, query 1 will equal query 2 equal query 3 } } public static Expression<Func<TEntity, bool>> GetPredicateEqual<TEntity>(string fieldName, object fieldValue) where TEntity : class { ParameterExpression m = Expression.Parameter(typeof(TEntity), "t"); var p = m.Type.GetProperty(fieldName); BinaryExpression body = Expression.Equal( Expression.Property(m, fieldName), Expression.Constant(fieldValue, p.PropertyType) ); return Expression.Lambda<Func<TEntity, bool>>(body, m); } public static Expression<Func<T, TReturn>> GetSelector<T, TReturn>(string fieldName) where T : class where TReturn : class { var t = typeof(TReturn); ParameterExpression p = Expression.Parameter(typeof(T), "t"); var body = Expression.Property(p, fieldName); return Expression.Lambda<Func<T, TReturn>>(body, new ParameterExpression[] { p }); } </code></pre>
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1. POUsing the Select method for dynamic queries and expression trees
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1. USLeng Weh Seng
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