Dynamically select LINQ SELECT fields at runtime

Question

I have a LINQ query to a database, with multiple linked tables, and need to return (SELECT) different fields depending on inputs.

ClassA has ParamA, ParamB, and          


        

Answer 1

This may be a bit verbose, but Expressions can get that way, for example:

var typeClassA = typeof(ClassA);
var typeClassB = typeof(ClassB);

var parameterExpressionP = Expression.Parameter(typeClassB, "p");
var newExpression = Expression.New(typeClassB);

var paramDPropertyExpression = Expression.Property(parameterExpressionP, "ParamD");
var paramDMemberBinding = Expression.Bind(typeClassB.GetProperty("ParamD"), paramDPropertyExpression);

var paramEPropertyExpression = Expression.Property(parameterExpressionP, "ParamE");
var paramEMemberBinding = Expression.Bind(typeClassB.GetProperty("ParamE"), paramEPropertyExpression);

var memberInitExpression = Expression.MemberInit(
    newExpression,
    paramDMemberBinding, paramEMemberBinding);

var projectionExpression = Expression.Lambda<Func<ClassB, ClassB>>(memberInitExpression, parameterExpressionP);

var parameterExpressionC = Expression.Parameter(typeClassA, "c");
var selectParamExpression = Expression.Property(parameterExpressionC, "ClassBs");

var selectExpression = Expression.Call(
    typeof(Enumerable),
    nameof(Enumerable.Select),
    new[] { typeClassB, typeClassB },
    selectParamExpression, projectionExpression);

var toListExpression = Expression.Call(
    typeof(Enumerable),
    nameof(Enumerable.ToList),
    new[] { typeClassB },
    selectExpression);

This will create an expression something like:

c.ClassBs.Select(p => new ClassB() {ParamD = p.ParamD, ParamE = p.ParamE}).ToList()