Dynamically building an expression tree for an EF Where() clause
I want to build a custom Where expression that I can pass to the Where clause of an Entity Framework query.
Below is as far as I’ve got.
// ??? ParameterExpression pe = Expression.Parameter(typeof(StorageDetail), "d"); if (HasRailcarNumber.HasValue) { Expression left = Expression.Property(pe, typeof(StorageDetail).GetProperty("RailcarNumber")); Expression right = Expression.Constant(null); Expression e2 = (HasRailcarNumber.Value == true) ? Expression.Equal(left, right) : Expression.NotEqual(left, right); // What to do with e2 ??? } if (IsTakeOrPay.HasValue) { Expression left = Expression.Property(pe, typeof(StorageDetail).GetProperty("TakeOrPayStartDate")); Expression right = Expression.Constant(null); Expression e2 = (HasRailcarNumber.Value == true) ? Expression.Equal(left, right) : Expression.NotEqual(left, right); // What to do with e2 ??? } if (HasArrived.HasValue) { Expression left = Expression.Property(pe, typeof(StorageDetail).GetProperty("ArrivalDate")); Expression right = Expression.Constant(null); Expression e2 = (HasRailcarNumber.Value == true) ? Expression.Equal(left, right) : Expression.NotEqual(left, right); // What to do with e2 ??? } My first question is how do I start the body? I don’t want my expresion to call Where(). I want Where to call my expression.
Second question is once I have my subexpressions (e2 above), how do I combine them? (Using AndAlso.) Note that it’s possible that all three properties are null, in which case the expression should not filter anything (should be a null expression).
As you’ve assumed you should use Expression.AndAlso for AND logic. If it is the case, you can create "start" expression evaluating to true and add others to it:
Expression expr = Expression.Constant(true); if (HasRailcarNumber.HasValue) { ... expr = Expression.AndAlso(expr, e2); } if (IsTakeOrPay.HasValue) { ... expr = Expression.AndAlso(expr, e2); } if (HasArrived.HasValue) { ... expr = Expression.AndAlso(expr, e2); } 
