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POHow do I nest a call to a method within a call to a method using Expression.Call
 

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  1. POHow do I nest a call to a method within a call to a method using Expression.Call
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    <p>I am trying to convert a DateTime to a String before calling contains on it. However despite my efforts of putting the result of one expression into another I fail miserably. </p> <p>The code is derived from the highest answer to this <a href="https://stackoverflow.com/questions/2413032/jqgrid-with-asp-net-webmethod-and-json-working-with-sorting-paging-searching-an" title="jqgrid with asp.net webmethod and json working with sorting, paging, searching and LINQ — but needs dynamic operators">question</a> jqgrid with asp.net webmethod and json working with sorting, paging, searching and LINQ — but needs dynamic operators.</p> <p>Assume I have the following method as StringExtension from the <a href="https://stackoverflow.com/questions/2413032/jqgrid-with-asp-net-webmethod-and-json-working-with-sorting-paging-searching-an" title="jqgrid with asp.net webmethod and json working with sorting, paging, searching and LINQ — but needs dynamic operators">question</a>:</p> <pre><code>public static class StringExtensions { public static MemberExpression ToMemberExpression(this string source, ParameterExpression p) { if (p == null) throw new ArgumentNullException("p"); string[] properties = source.Split('.'); Expression expression = p; Type type = p.Type; foreach (var prop in properties) { var property = type.GetProperty(prop); if (property == null) throw new ArgumentException("Invalid expression", "source"); expression = Expression.MakeMemberAccess(expression, property); type = property.PropertyType; } return (MemberExpression)expression; } } </code></pre> <p>Therefore I have the following method also from the <a href="https://stackoverflow.com/questions/2413032/jqgrid-with-asp-net-webmethod-and-json-working-with-sorting-paging-searching-an" title="jqgrid with asp.net webmethod and json working with sorting, paging, searching and LINQ — but needs dynamic operators">question</a> which I have then adapted for DateTime.</p> <pre><code>public virtual Expression&lt;Func&lt;T, bool&gt;&gt; CreateExpression&lt;T&gt;(string searchField, string searchString, string searchOper) { Expression exp = null; var p = Expression.Parameter(typeof(T), "p"); Expression propertyAccess = searchField.ToMemberExpression(p); switch (searchOper) { case "bw": exp = Expression.Call(propertyAccess, typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) }), Expression.Constant(searchString)); break; // New code by me case "cn": if (propertyAccess.Type == typeof(DateTime)) { // My faulty logic - from Jon Skeet answer below Expression toStringCall = Expression.Call( propertyAccess, "ToString", null, new[] { Expression.Constant("D") }); Expression containsCall = Expression.Call( toStringCall, "Contains", null, new[] { Expression.Constant(searchString) }); exp = containsCall; } else { // Unchanged exp = Expression.Call(propertyAccess, typeof(string).GetMethod("Contains", new Type[] { typeof(string) }), Expression.Constant(searchString)); } break; case "ew": exp = Expression.Call(propertyAccess, typeof(string).GetMethod("EndsWith", new Type[] { typeof(string) }), Expression.Constant(searchString)); break; case "gt": exp = Expression.GreaterThan(propertyAccess, Expression.Constant(searchString, propertyAccess.Type)); break; case "ge": exp = Expression.GreaterThanOrEqual(propertyAccess, Expression.Constant(searchString, propertyAccess.Type)); break; case "lt": exp = Expression.LessThan(propertyAccess, Expression.Constant(searchString, propertyAccess.Type)); break; case "le": exp = Expression.LessThanOrEqual(propertyAccess, Expression.Constant(searchString, propertyAccess.Type)); break; case "eq": exp = Expression.Equal(propertyAccess, Expression.Constant(searchString.ToType(propertyAccess.Type), propertyAccess.Type)); break; case "ne": exp = Expression.NotEqual(propertyAccess, Expression.Constant(searchString, propertyAccess.Type)); break; default: return null; } return (Expression&lt;Func&lt;T, bool&gt;&gt;)Expression.Lambda(exp, p); } </code></pre> <p>I get the following exception.</p> <p>LINQ to Entities does not recognize the method 'System.String ToString(System.String)' method, and this method cannot be translated into a store expression.</p>
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