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    <p>AFAIK you can only join this way:</p> <pre><code>var query = from obj_i in set1 join obj_j in set2 on new { JoinProperty1 = obj_i.SomeField1, JoinProperty2 = obj_i.SomeField2, JoinProperty3 = obj_i.SomeField3, JoinProperty4 = obj_i.SomeField4 } equals new { JoinProperty1 = obj_j.SomeOtherField1, JoinProperty2 = obj_j.SomeOtherField2, JoinProperty3 = obj_j.SomeOtherField3, JoinProperty4 = obj_j.SomeOtherField4 } </code></pre> <p><strong>The main requirements are: Property names, types and order in the anonymous objects your'e joining on must match.</strong></p> <p>You CAN'T use ANDs ORs etc. in joins. Just object1 equals object2.</p> <p>More advanced stuff in this LinqPad example:</p> <pre><code>class c1 { public int someIntField; public string someStringField; } class c2 { public Int64 someInt64Property {get;set;} private object someField; public string someStringFunction(){return someField.ToString();} } void Main() { var set1 = new List&lt;c1&gt;(); var set2 = new List&lt;c2&gt;(); var query = from obj_i in set1 join obj_j in set2 on new { JoinProperty1 = (Int64) obj_i.someIntField, JoinProperty2 = obj_i.someStringField } equals new { JoinProperty1 = obj_j.someInt64Property, JoinProperty2 = obj_j.someStringFunction() } select new {obj1 = obj_i, obj2 = obj_j}; } </code></pre> <p>Addressing names and property order is straightforward, addressing types can be achieved via casting/converting/parsing/calling methods etc. This might not always work with LINQ to EF or SQL or NHibernate, most method calls definitely won't work and will fail at run-time, so YMMV. This is because they are copied to public read-only properties in the anonymous objects, so as long as your expression produces values of correct type the join property - you should be fine.</p>
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    1. POLINQ Joining in C# with multiple conditions
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    1. USZar Shardan
    1. USZar Shardan
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    1. CO+1 for best answer here
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      1. USericosg
    2. COpretty sure the field names need to match, you get a type inference error if they dont
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      1. USDaniel Powell
    3. COThanks Daniel, you are correct, updated the answer to reflect your notes and added another more advanced example.
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