This question is best answered as a syllogism, checked premise by premise against the case law, rather than as a list of precedents.
Major premise.
Once S.R. Bommai v. Union of India (1994) held that the loss of a government's majority in the Assembly is a fact that can only be established by a floor test, and not by the Governor's subjective opinion, a floor test became the exclusive constitutional method for establishing that fact whenever the question of majority genuinely arises.
Minor premise.
Dissolving the Assembly before conducting a floor test removes the only forum in which majority can be constitutionally established, and substitutes the Governor's own assessment, based on letters, defections or media reports, for the test Bommai requires.
Conclusion.
If a floor test is the exclusive method of establishing majority, and pre floor test dissolution forecloses that method entirely, then pre floor test dissolution is invalid whenever the underlying justification for it is a claim about lost majority.
Confirming the premises against the case law.
Rameshwar Prasad v. Union of India (2006) applied exactly this reasoning to strike down the dissolution of the Bihar Assembly, holding that the Governor had acted on unverified claims rather than any floor test and had acted mala fide. Nabam Rebia v. Deputy Speaker (2016) reinforced the same point in the Arunachal Pradesh crisis, holding that the Governor cannot act on personal satisfaction alone and must have objective material, with a floor test as the natural way to generate that material. Later political crises before the Election Commission kept returning to the same rule: majority is proved in the House, not asserted outside it.
The narrow exception that keeps the conclusion honest.
The syllogism assumes the question of majority "genuinely arises" as a live, testable question. If the Assembly itself becomes non functional, for instance through a mass resignation that leaves no quorum to conduct any test, there is nothing left for a floor test to establish, and dissolution may be the only option left. This is a genuine exception, not a loophole, because it applies only when a floor test has become physically impossible rather than merely inconvenient for the ruling side.
Outside that narrow situation, the syllogism holds without qualification: dissolving the Assembly before a floor test defeats the one method the Constitution recognises for testing majority, and is therefore invalid.
\[ \boxed{\text{Generally invalid; valid only where a floor test has become genuinely impossible}} \]