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  1. PO
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    <p>Why don't you simply replace <code>T1</code> and <code>T2</code> with a single generic type parameter? Then the second parameters type needn't be inferred - which it doesn't have to be anyhow since it must be identical to that of the first parameter.</p> <p>That would look like this:</p> <h2>Variant 1</h2> <pre><code>public KeyValuePair&lt;Expression&lt;Func&lt;Task, T&gt;&gt;, T&gt; Marry&lt;T&gt;( Expression&lt;Func&lt;Task, T&gt;&gt; key, T value) { return new KeyValuePair&lt;Expression&lt;Func&lt;Task, T&gt;&gt;, T&gt;(key, value); } </code></pre> <p>I don't recommend this, but if you want the type-inference to "greedily" determine the type of the expression based solely on the first parameter (so that you get the error "cannot convert from 'string' to 'DateTime?'), you can curry the method:</p> <h2>Variant 2</h2> <pre><code>public static Func&lt;T, KeyValuePair&lt;Expression&lt;Func&lt;Task, T&gt;&gt;, T&gt; &gt; Marry&lt;T&gt;( Expression&lt;Func&lt;Task, T&gt;&gt; key) { return (value=&gt;new KeyValuePair&lt;Expression&lt;Func&lt;Task, T&gt;&gt;, T&gt;(key, value)); } //usage: Marry(t =&gt; t.NULLABLEDATETIME)("test"); //error: Delegate 'System.Func&lt;System.DateTime?,System.Collections.Generic.KeyValuePair&lt;System.Linq.Expressions.Expression&lt;System.Func&lt;UserQuery.Task,System.DateTime?&gt;&gt;,System.DateTime?&gt;&gt;' has some invalid arguments // - Argument 1: cannot convert from 'string' to 'System.DateTime?' </code></pre> <p>That error message essentially covers it. However, the API isn't too readable and the usage of heavily nested generic types makes things... less readable. A longer variant with essentially the same behavior but shorter error messages could look as follows:</p> <h2>Variant 3</h2> <pre><code>public class MarriagePartner&lt;T&gt; { readonly Expression&lt;Func&lt;Task, T&gt;&gt; key; public MarriagePartner(Expression&lt;Func&lt;Task, T&gt;&gt; key) { this.key = key; } public KeyValuePair&lt;Expression&lt;Func&lt;Task, T&gt;&gt;, T&gt; With(T value) { return new KeyValuePair&lt;Expression&lt;Func&lt;Task, T&gt;&gt;, T&gt;(key, value); } } public static MarriagePartner&lt;T&gt; Marry2&lt;T&gt;(Expression&lt;Func&lt;Task, T&gt;&gt; key) { return new MarriagePartner&lt;T&gt;(key); } //Usage: Marry2(t =&gt; t.NULLABLEDATETIME).With("test"); //error: The best overloaded method match for 'UserQuery.MarriagePartner&lt;System.DateTime?&gt;.With(System.DateTime?)' has some invalid arguments // - Argument 1: cannot convert from 'string' to 'System.DateTime?' </code></pre> <p>That error message is a little more specific. I think Variant 1's error message suffices and it's what I'd use (KISS and all) - but if you <em>Really</em> want that casting error message, Variant 3 has the most understandable (if longer) implementation and most friendly error message.</p>
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    1. POChecking if some function parameters data types are equal due compilation on C#
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    1. USEamon Nerbonne
    1. USEamon Nerbonne
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    1. COBecause now when call function with wrong parameters Marry(t => t.NULLABLEDATETIME, "SomeString"); compiler says: The type arguments for method 'Lambda.Db.Task.Task.Marry<T>(System.Linq.Expressions.Expression<System.Func<Task,T>>, T)' cannot be inferred from the usage. Try specifying the type arguments explicitly. As you can see, we still have this message. I would like compiler to say: There is no implicit reference conversion from 'string' to 'DateTime?'.
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      1. USOleg Mazko
    2. COIt's not as serious an error, however. With the T1+T2 method, correct cases (informally: T1 string, T2 null) can't be resolved, whereas with this method, although the error message may not be ideal, more correct cases work and incorrect cases don't.
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      1. USEamon Nerbonne
    3. COAlso, the error message you're seeing is standard for linq. For instance, try: `Enumerable.Concat(new[]{3},new[]{"test"});` which results in the same error message.
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      1. USEamon Nerbonne
 

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