Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>It seems like if we could dynamically generate the equivalent of the expression: x => new { a = x.Prop1, b = x.Prop2, ... } Then a reasonable LINQ => SQL provider would probably generate the SQL you want. Generating a new anonymous type on the fly seems hard, but we could take advantage of the fact that anonymous types are generic to re-use one.</p> <pre><code>// at the top of your class private static readonly object AnonymousObjectWithLotsOfProperties = new { a = 1, b = 2, ... }; // then in your method // (1) first get the list of properties represented by the string you passed in (I assume you know how to do this via reflection) var props = typeof(T).GetProperties().Where(...); var propTypes = props.Select(pi =&gt; pi.PropertyType).ToList(); // (2) now generate the correctly genericized anonymous type to use var genericTupleType = AnonymousObjectWithLotsOfProperties.GetType() .GetGenericTypeDefinition(); // for generic args, use the prop types and pad with int var genericArguments = propTypes.Concat(Enumerable.Repeat(typeof(int), genericTupleType.GetProperties().Length - propTypes.Count)) .ToArray(); var tupleType = genericTupleType.MakeGenericType(genericArguments); // (3) now we have to generate the x =&gt; new { ... } expression // if you inspect "System.Linq.Expressions.Expression&lt;Func&lt;object&gt;&gt; exp = () =&gt; new { a = 2, b = 3 };" // in the VS debugger, you can see that this is actually a call to a constructor // on the anonymous type with 1 argument per property var tParameter = Expression.Parameter(typeof(T)); // create the expression var newExpression = Expression.New( constructor: tupleType.GetConstructors().Single(), // get the one constructor we need // the arguments are member accesses on the parameter of type T, padded with 0 arguments: props.Select(pi =&gt; Expression.MakeMemberAccess(tParameter, pi)) .Concat(Enumerable.Repeat(Expression.Constant(0), genericTupleType.GetProperties().Length - propTypes.Count)) ); // create the lambda: we need an Expression&lt;TDelegate&gt;, which means that we // need to get the generic factory method from Expression and invoke it var lambdaGenericMethod = typeof(Expression).GetMethods(BindingFlags.Static | BindingFlags.Public) .Single(m =&gt; m.IsGenericMethodDefinition); var lambdaMethod = lambdaGenericMethod.MakeGenericMethod(typeof(Func&lt;,&gt;).MakeGenericType(typeof(T), tupleType)); // reflection for Expression.Lambda(body, parameters) var lambda = lambdaGenericMethod.Invoke(null, new object[] { newExpression, new[] { tParameter }); // now that we have the expression, we can invoke GroupBy via reflection. // Of course, that leaves you with an IQueryable&lt;IGrouping&lt;ANON, T&gt;&gt;, which isn't much // use until you apply some other IQueryable methods to eliminate the ANON type from the // method signature so you can return it </code></pre> <p>Note that I didn't actually get to compile and run this code, so I can't promise that everything above is perfect. However, hopefully it will put you on the right track.</p>
    singulars
    1. This table or related slice is empty.
    1. POMultiple Groupby keySelectors using Linq Expressions
      singulars
      1. PTQuestion
    1. PTAnswer
    1. This table or related slice is empty.
    1. USChaseMedallion
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. COThanks Chase for your response. I was looking for a solution to create keyselector with anonymous types representing the group by columns. Your code gives me a good headstart. I get the error Late bound operations cannot be performed on types or methods for which ContainsGenericParameters is true in the lamdaGenericMethod.Invoke line .. I am trying to solve this right now ... Thanks
      singulars
      1. PO
      1. USideafountain
    2. CO@ideafountain that error just means that you're trying to invoke a GenericMethodDefinition (e. g. Select<>() rather than Select<int>()). Where in the code are you hitting this?
      singulars
      1. PO
      1. USChaseMedallion
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload